Display device and electronic device

ABSTRACT

A highly reliable display device. A first flexible substrate and a second flexible substrate overlap each other with a display element positioned therebetween. Side surfaces of at least one of the first substrate and the second substrate which overlap each other are covered with a high molecular material having a light-transmitting property. The high molecular material is more flexible than the first substrate and the second substrate.

This application is a continuation of copending U.S. application Ser.No. 15/865,482, filed on Jan. 9, 2018 which is a continuation of U.S.application Ser. No. 14/806,271, filed on Jul. 22, 2015 (now U.S. Pat.No. 9,876,059 issued Jan. 23, 2018) which are all incorporated herein byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a display device. Oneembodiment of the present invention also relates to a method formanufacturing the display device.

Note that one embodiment of the present invention is not limited to theabove technical field. For example, one embodiment of the presentinvention relates to an object, a method, or a manufacturing method. Oneembodiment of the present invention relates to a process, a machine,manufacture, or a composition of matter. One embodiment of the presentinvention relates to a memory device, a processor, a driving methodthereof, or a manufacturing method thereof.

Note that in this specification and the like, a semiconductor devicegenerally means a device that can function by utilizing semiconductorcharacteristics. Thus, a semiconductor element such as a transistor or adiode and a semiconductor circuit are semiconductor devices. A displaydevice, a light-emitting device, a lighting device, an electro-opticaldevice, an electronic device, and the like may include a semiconductorelement or a semiconductor circuit. Therefore, a display device, alight-emitting device, a lighting device, an electro-optical device, animaging device, an electronic device, and the like include asemiconductor device in some cases.

BACKGROUND ART

In recent years, research and development have been extensivelyconducted on liquid crystal elements as a display element used in adisplay region of a display device. In addition, research anddevelopment have been extensively conducted on light-emitting elementsutilizing electroluminescence (EL). As a basic structure of theselight-emitting elements, a layer containing a light-emitting substanceis provided between a pair of electrodes. Voltage is applied to thislight-emitting element to obtain light emission from the light-emittingsubstance.

Light-emitting elements are a self-luminous element; thus, a displaydevice using the light-emitting elements has, in particular, advantagessuch as high visibility, no necessity of a backlight, and low powerconsumption. The display device using the light-emitting elements alsohas advantages in that it can be manufactured to be thin and lightweightand has high response speed.

A display device including the display elements can have flexibility;therefore, the use of a flexible substrate for the display device hasbeen proposed.

As a method for manufacturing a display device using a flexiblesubstrate, a technique is known in which a semiconductor element such asa thin film transistor is manufactured over a substrate such as a glasssubstrate or a quartz substrate, for example, the semiconductor elementis fixed to another substrate (e.g., a flexible substrate) by using anorganic resin, and then the semiconductor element is transferred fromthe glass substrate or the quartz substrate to the other substrate(Patent Document 1).

In addition, a technique for enhancing the mechanical strength of adisplay device by sandwiching an organic EL panel formed using a glasssubstrate with a thickness of greater than or equal to 20 μm and lessthan or equal to 50 μm between two flexible sheets is known (PatentDocument 2).

Display devices are expected to be applied to a variety of uses andbecome diversified. For example, a smartphone and a tablet terminal witha touch panel are being developed as portable information terminals.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2003-174153

[Patent Document 2] Japanese Published Patent Application No.2010-244694

DISCLOSURE OF INVENTION

To protect a surface of a light-emitting element and prevent entry ofimpurity, such as moisture, from the outside, an additional substrate isattached to a light-emitting element formed over a substrate in somecases. However, there is a problem in that impurity such as moisturethat enters from the outer periphery of the attached substrates (an edgeof the substrates) contributes to a decrease in display quality and adecrease in reliability. To avoid this problem, the conventional displaydevice needs a long distance from an edge of a substrate to a displayregion. As a result, a region that is outer than the display region andthat does not contribute to display (hereinafter also referred to asframe) is wide, which inhibits an improvement in the productivity or thedesign flexibility of a display device and a semiconductor deviceincluding the display device.

Moreover, in the case where an organic EL panel is sandwiched betweentwo flexible sheets as disclosed in Patent Document 2, impurity thatenters an edge of the flexible sheets may cause a deterioration indisplay image or a decrease in reliability. In addition, in PatentDocument 2, the flexible sheets are larger than the organic EL panel,which inevitably widens the frame including the flexible sheets.

An object of one embodiment of the present invention is to provide ahighly reliable display device and a method for manufacturing thedisplay device. Another object of one embodiment of the presentinvention is to provide a display device with high design flexibilityand a method for manufacturing the display device.

Another object of one embodiment of the present invention is to providea display device, electronic device, or the like having high visibility.Another object of one embodiment of the present invention is to providea display device, electronic device, or the like having high displayquality. Another object of one embodiment of the present invention is toprovide a display device, electronic device, or the like having highreliability. Another object of one embodiment of the present inventionis to provide a display device, electronic device, or the like that isunlikely to be broken.

Another object of one embodiment of the present invention is to providea display device, electronic device, or the like with low powerconsumption. Another object of one embodiment of the present inventionis to provide a display device, electronic device, or the like with highproductivity. Another object of one embodiment of the present inventionis to provide a novel display device, electronic device, or the like.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all of these objects. Other objects will be apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a display device including afirst substrate, a second substrate, and a first layer. The firstsubstrate and the second substrate overlap each other with a displayelement positioned therebetween. The first layer covers the firstsubstrate in a region where the first substrate and the second substrateoverlap each other, the second substrate in the region where the firstsubstrate and the second substrate overlap each other, a side surface ofthe first substrate, and a side surface of the second substrate.

For the first layer, a silicone resin (silicone rubber) or a fluorineresin (fluorine rubber) may be used, for example. The first layer can beformed by providing a liquid high molecular material on the periphery ofthe first substrate and the second substrate which overlap each otherand curing the liquid high molecular material. Since the first layer isseamless, by covering the outer periphery (an edge of substrates) of theoverlapped first and second substrates with the first layer, impuritysuch as moisture can be prevented from entering a display region.

Another embodiment of the present invention is a display deviceincluding a first substrate, a second substrate, and a first layer. Thefirst substrate and the second substrate overlap each other with adisplay element positioned therebetween. The first layer covers thefirst substrate in a region where the first substrate and the secondsubstrate overlap each other, the second substrate in a region where thefirst substrate and the second substrate overlap each other, and atleast one of a side surface of the first substrate and a side surface ofthe second substrate.

One embodiment of the present invention can provide a highly reliabledisplay device and a manufacturing method thereof. Another embodiment ofthe present invention can provide a display device with high designflexibility and a manufacturing method thereof.

One embodiment of the present invention provides a display device,electronic device, or the like having high visibility. One embodiment ofthe present invention provides a display device, electronic device, orthe like having high display quality. One embodiment of the presentinvention provides a display device, electronic device, or the likehaving high reliability. One embodiment of the present inventionprovides a display device, electronic device, or the like that isunlikely to be broken. One embodiment of the present invention providesa display device, electronic device, or the like with low powerconsumption. One embodiment of the present invention provides a displaydevice, electronic device, or the like with high productivity. Oneembodiment of the present invention provides a novel display device,electronic device, or the like.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all of these effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one embodiment of a display device.

FIGS. 2A to 2C are a plan view and cross-sectional views illustratingone embodiment of a display device.

FIG. 3 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 4A to 4C are a block diagram and circuit diagrams illustratingembodiments of a display device.

FIGS. 5A and 5B are block diagrams illustrating embodiments of a displaydevice.

FIGS. 6A and 6B each illustrate an example of a pixel configuration ofone embodiment of a display device.

FIGS. 7A and 7B each illustrate an example of a pixel configuration ofone embodiment of a display device.

FIGS. 8A to 8D are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 9A to 9D are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 10A to 10D are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 11A and 11B are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 12A and 12B are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 13A and 13B are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 14A and 14B are cross-sectional views illustrating an example of amethod for fabricating a display device.

FIGS. 15A and 15B each illustrate one embodiment of a display device.

FIGS. 16A to 16C illustrate an example of a method for fabricating adisplay device.

FIG. 17 illustrates an example of a method for fabricating a displaydevice.

FIGS. 18A1 and 18A2 and FIGS. 18B1 to 18B3 illustrate embodiments of adisplay device.

FIG. 19 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 20A and 20B are cross-sectional views illustrating one embodimentof a display device.

FIG. 21 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 22A1, 22A2, 22B1, 22B2, 22C1, and 22C2 each illustrate oneembodiment of a transistor.

FIGS. 23A1 to 23A3, 23B1, and 23B2 each illustrate one embodiment of atransistor.

FIGS. 24A1 to 24A3, 24B1, 24B2, 24C1, and 24C2 each illustrate oneembodiment of a transistor.

FIGS. 25A to 25C illustrate one embodiment of a transistor.

FIGS. 26A to 26C illustrate one embodiment of a transistor.

FIGS. 27A to 27C illustrate one embodiment of a transistor.

FIGS. 28A to 28C illustrate one embodiment of a transistor.

FIG. 29 illustrates an energy band structure.

FIGS. 30A and 30B each illustrate a structure example of alight-emitting element.

FIGS. 31A to 31F illustrate examples of electronic devices and lightingdevices.

FIGS. 32A and 32B illustrate an example of an electronic device.

FIGS. 33A to 33C illustrate an example of an electronic device.

FIGS. 34A to 34I illustrate examples of electronic devices.

FIGS. 35A and 35B illustrate an example of an electronic device.

FIG. 36 illustrates an example of an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to theaccompanying drawings. Note that the present invention is not limited tothe description below, and it is understood easily by those skilled inthe art that various changes and modifications can be made withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thedescription in the following embodiments. In the structures of thepresent invention to be described below, the same portions or portionshaving similar functions are denoted by the same reference numerals indifferent drawings, and explanation thereof will not be repeated.

The position, size, range, and the like of each component illustrated inthe drawings and the like are not accurately represented in some casesto facilitate understanding of the invention. Therefore, the disclosedinvention is not necessarily limited to the position, the size, range,and the like disclosed in the drawings and the like. For example, in theactual manufacturing process, a resist mask or the like might beunintentionally reduced in size by treatment such as etching, whichmight not be illustrated for easy understanding.

Especially in a top view (also referred to as a plan view), aperspective view, or the like, some components might not be illustratedfor easy understanding.

In this specification and the like, the term such as an “electrode” or a“wiring” does not limit a function of a component. For example, an“electrode” is used as part of a “wiring” in some cases, and vice versa.Furthermore, the term “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

Note that the term “over” or “under” in this specification and the likedoes not necessarily mean that a component is placed “directly on” or“directly below” and “directly in contact with” another component. Forexample, the expression “electrode B over insulating layer A” does notnecessarily mean that the electrode B is on and in direct contact withthe insulating layer A and can mean the case where another component isprovided between the insulating layer A and the electrode B.

Functions of a source and a drain might be switched depending onoperation conditions, for example, when a transistor having oppositepolarity is employed or the direction of current flow is changed incircuit operation. Thus, it is difficult to define which is a source ora drain. Accordingly, the terms “source” and “drain” can be switched inthis specification.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Accordingly, even whenthe expression “electrically connected” is used in this specification,there is a case in which no physical connection is made and a wiring isjust extended in an actual circuit.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Aterm “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly also includes the case where the angle is greaterthan or equal to 85° and less than or equal to 95°.

In this specification, in the case where an etching step is performedafter a lithography process, a resist mask formed in the lithographyprocess is removed after the etching step, unless otherwise specified.

A voltage usually refers to a potential difference between a givenpotential and a reference potential (e.g., a source potential or aground potential (a GND potential)). A voltage can be referred to as apotential and vice versa.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration lower than 0.1 atomic % can beregarded as an impurity. When an impurity is contained, the density ofstates (DOS) in a semiconductor may be increased, the carrier mobilitymay be decreased, or the crystallinity may be decreased, for example. Inthe case where the semiconductor is an oxide semiconductor, examples ofan impurity which changes characteristics of the semiconductor includeGroup 1 elements, Group 2 elements, Group 13 elements, Group 14elements, Group 15 elements, and transition metals other than the maincomponents of the oxide semiconductor; specifically, there are hydrogen(included in water), lithium, sodium, silicon, boron, phosphorus,carbon, and nitrogen, for example. In the case of an oxidesemiconductor, oxygen vacancies may be formed by entry of impuritiessuch as hydrogen. In the case where the semiconductor is silicon,examples of an impurity which changes characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Note that ordinal numbers such as “first” and “second” in thisspecification and the like are used in order to avoid confusion amongcomponents and do not denote the priority or the order such as the orderof steps or the stacking order. A term without an ordinal number in thisspecification and the like might be provided with an ordinal number in aclaim in order to avoid confusion among components. A term with anordinal number in this specification and the like might be provided witha different ordinal number in a claim. Moreover, a term with an ordinalnumber in this specification and the like might not be provided with anyordinal number in a claim.

Note that in this specification, the channel length refers to, forexample, a distance, observed in a top view of a transistor, between asource (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor and agate electrode overlap with each other, a portion where a current flowsin a semiconductor when the transistor is on, or a region where achannel is formed. In one transistor, channel lengths are notnecessarily the same in all regions. In other words, the channel lengthof one transistor is not limited to one value in some cases. Therefore,in this specification, the channel length is any one of values, themaximum value, the minimum value, or the average value in a region wherea channel is formed.

Note that in this specification and the like, an “on state” of atransistor refers to a state in which a source and a drain of thetransistor are electrically short-circuited (also referred to as being“brought into conduction”). Furthermore, an “off state” of thetransistor refers to a state in which the source and the drain of thetransistor are electrically disconnected (also referred to as being“brought out of conduction”.)

In this specification and the like, in some cases, “on-state current”means a current which flows between a source and a drain when atransistor is on, and “off-state current” means a current which flowsbetween a source and a drain when a transistor is off.

The off-state current of a transistor depends on a voltage between agate and a source (also referred to as Vgs) in some cases. Thus, “theoff-state current of a transistor is lower than or equal to I” means“there is Vgs with which the off-state current of the transistor becomeslower than or equal to I” in some cases. The off-state current of atransistor may refer to a current at a certain Vgs or a current at Vgsin a certain voltage range.

As an example, the assumption is made of an n-channel transistor wherethe threshold voltage Vth is 0.5 V and the current flowing between asource and a drain (hereinafter also referred to as Ids) is 1×10⁻⁹ A atVgs of 0.5 V, 1×10⁻¹³ A at Vgs of 0.1 V, 1×10⁻¹⁹ A at Vgs of −0.5 V, and1×10⁻²² A at Vgs of −0.8 V. The Ids of the transistor is 1×10⁻¹⁹ A orlower at Vgs of −0.5 V or at Vgs in the range of −0.8 V to −0.5 V;therefore, it can be said that the off-state current of the transistoris 1×10⁻¹⁹ A or lower. Since there is Vgs at which the drain current ofthe transistor is 1×10⁻²² A or lower, it can be said that the off-statecurrent of the transistor is 1×10⁻²² A or lower.

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be an off-state current at room temperature, 60° C.,85° C., 95° C., or 125° C. Alternatively, the off-state current may bean off-state current at a temperature at which the reliability of asemiconductor device or the like including the transistor is ensured ora temperature at which the semiconductor device or the like is used(e.g., temperature in the range of 5° C. to 35° C.). When there is Vgsat which the off-state current of a transistor at room temperature, 60°C., 85° C., 95° C., 125° C., a temperature at which the reliability of asemiconductor device or the like including the transistor is ensured, ora temperature at which the semiconductor device or the like is used(e.g., temperature in the range of 5° C. to 35° C.) is lower than orequal to I, it may be said that the off-state current of the transistoris lower than or equal to I.

The off-state current of a transistor depends on voltage between itsdrain and source (hereinafter also referred to as Vds) in some cases.Unless otherwise specified, the off-state current in this specificationmay be an off-state current at Vds with an absolute value of 0.1 V, 0.8V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V.Alternatively, the off-state current may be an off-state current at Vdsat which the reliability of a semiconductor device or the like includingthe transistor is ensured or Vds used in the semiconductor device or thelike. When there is Vgs at which the off-state current of a transistoris lower than or equal to I at given Vds, it may be said that theoff-state current of the transistor is lower than or equal to I. Here,given Vds is, for example, 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V,3.3 V, 10 V, 12 V, 16 V, 20 V, Vds at which the reliability of asemiconductor device or the like including the transistor is ensured, orVds used in the semiconductor device or the like.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductorand a gate electrode overlap with each other, a portion where a currentflows in a semiconductor when a transistor is on, or a region where achannel is formed. In one transistor, channel widths are not necessarilythe same in all regions. In other words, the channel width of onetransistor is not limited to one value in some cases. Therefore, in thisspecification, a channel width is any one of values, the maximum value,the minimum value, or the average value in a region where a channel isformed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having a gateelectrode covering a side surface of a semiconductor, an effectivechannel width is greater than an apparent channel width, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a gate electrode covering a side surfaceof a semiconductor, the proportion of a channel region formed in a sidesurface of a semiconductor is higher than the proportion of a channelregion formed in a top surface of a semiconductor in some cases. In thatcase, an effective channel width is greater than an apparent channelwidth.

In such a case, an effective channel width is difficult to measure insome cases. For example, to estimate an effective channel width from adesign value, it is necessary to assume that the shape of asemiconductor is known as an assumption condition. Therefore, in thecase where the shape of a semiconductor is not known accurately, it isdifficult to measure an effective channel width accurately.

Therefore, in this specification, an apparent channel width is referredto as a surrounded channel width (SCW) in some cases. Furthermore, inthis specification, in the case where the term “channel width” is simplyused, it may denote a surrounded channel width and an apparent channelwidth. Alternatively, in this specification, in the case where the term“channel width” is simply used, it may denote an effective channel widthin some cases. Note that a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by analyzing a cross-sectional TEM imageand the like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are calculated, a surroundedchannel width might be used for the calculation. In that case, a valuemight be different from one calculated by using an effective channelwidth.

Embodiment 1

A structure example of a display device 100 of one embodiment of thepresent invention is described with reference to FIG. 1, FIGS. 2A to 2C,FIG. 3, FIGS. 4A to 4C, FIGS. 5A and 5B, FIGS. 6A and 6B, and FIGS. 7Aand 7B. Note that the display device 100 disclosed in this specificationis a display device in which a light-emitting element is used as adisplay element. As the display device 100 of one embodiment of thepresent invention, a display device having a top-emission structure isdescribed as an example. Note that the display device 100 of oneembodiment of the present invention can be a display device having abottom-emission structure or a dual-emission structure.

<Structure of Display Device>

FIG. 1 is a perspective view of the display device 100 to which anexternal electrode 124 is connected and which is covered with the layer147. FIG. 2A is a plan view of the light-emitting device 100. FIG. 2B isa cross-sectional view taken along the dashed-dotted line V1-V2 in FIG.2A. FIG. 2C is a cross-sectional view taken along the dashed-dotted lineH1-H2 in FIG. 2A. FIG. 3 is a detailed cross-sectional view taken alongthe dashed-dotted line A1-A2 in FIG. 1. Note that FIG. 3 morespecifically illustrates part of the cross section in FIG. 2C.

The display device 100 described in this embodiment includes a displayregion 131, a circuit 132, and a circuit 133. The display device 100also includes a terminal electrode 216 and a light-emitting element 125including an electrode 115, an EL layer 117, and an electrode 118. Aplurality of light-emitting elements 125 are formed in the displayregion 131. A transistor 232 for controlling the amount of light emittedfrom the light-emitting element 125 is connected to each of thelight-emitting elements 125.

The external electrode 124 and the terminal electrode 216 areelectrically connected to each other through an anisotropic conductiveconnection layer 123. A part of the terminal electrode 216 iselectrically connected to the circuit 132, and another part of theterminal electrode 216 is electrically connected to the circuit 133.

The circuit 132 and the circuit 133 each include a plurality oftransistors 252. The circuit 132 and the circuit 133 each have afunction of determining which of the light-emitting elements 125 in thedisplay region 131 is supplied with a signal through the externalelectrode 124.

The transistor 232 and the transistor 252 each include a gate electrode206, a gate insulating layer 207, a semiconductor layer 208, a sourceelectrode 209 a, and a drain electrode 209 b. A wiring 219 is formed inthe same layer where the source electrode 209 a and the drain electrode209 b are formed. In addition, an insulating layer 210 is formed overthe transistor 232 and the transistor 252, and an insulating layer 211is formed over the insulating layer 210. The electrode 115 is formedover the insulating layer 211. The electrode 115 is electricallyconnected to the drain electrode 209 b through an opening formed in theinsulating layer 210 and the insulating layer 211. A partition 114 isformed over the electrode 115, and the EL layer 117 and the electrode118 are formed over the electrode 115 and the partition 114.

In the display device 100, a substrate 111 and a substrate 121 areattached to each other with a bonding layer 120 provided therebetween.

One surface of the substrate 111 is provided with an insulating layer205 with a bonding layer 112 positioned therebetween. One surface of thesubstrate 121 is provided with an insulating layer 145 with a bondinglayer 142 positioned therebetween. The one surface of the substrate 121is provided with a light-blocking layer 264 with the insulating layer145 positioned therebetween. The one surface of the substrate 121 isalso provided with a coloring layer 266 and an overcoat layer 268 withthe insulating layer 145 positioned therebetween.

The insulating layer 205 functions as a base layer and can prevent orreduce diffusion of moisture or impurity elements from the substrate111, the bonding layer 112, or the like to the transistor or thelight-emitting element. The insulating layer 145 functions as a baselayer and can prevent or reduce diffusion of moisture or impurityelements from the substrate 121, the bonding layer 142, or the like tothe transistor or the light-emitting element.

The insulating layer 205 and the insulating layer 145 are preferablyformed as a single layer or a multilayer using any of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, and aluminum nitride oxide. The insulatinglayer 205 and the insulating layer 145 can be formed by a sputteringmethod, a CVD method, a thermal oxidation method, a coating method, aprinting method, or the like.

For example, a flexible material such as an organic resin material canbe used for the substrate 111 and the substrate 121. In the case wherethe display device 100 has a bottom-emission structure or adual-emission structure, a material having a light-transmitting propertywith respect to light emitted from the EL layer 117 is used for thesubstrate 111. In the case where the display device 100 has atop-emission structure or a dual-emission structure, a material having alight-transmitting property with respect to light emitted from the ELlayer 117 is used for the substrate 121.

If the mechanical strength of a material used for the substrate 111 andthe substrate 121 is too low, the substrates easily become deformed atthe time of manufacture of the display device 100, which reduces yieldand thus, contributes to a reduction in productivity. Yet, if themechanical strength of the material used for the substrate 111 and thesubstrate 121 is too high, the display device becomes difficult to bend.An index of the mechanical strength of a material is a Young's modulus.The Young's modulus of a material suitable for the substrate 111 and thesubstrate 121 is larger than or equal to 1 GPa (1×10⁹ Pa) and smallerthan or equal to 100 GPa (100×10⁹ Pa), preferably larger than or equalto 2 GPa and smaller than or equal to 50 GPa, further preferably largerthan or equal to 2 GPa and smaller than or equal to 20 GPa. Note that inmeasurement of a Young's modulus, ISO527, JISK7161, JISK7162, JISK7127,ASTMD638, ASTMD882, or the like can be referred to.

The thickness of each of the substrate 111 and the substrate 121 ispreferably greater than or equal to 5 μm and less than or equal to 100μm, further preferably greater than or equal to 10 μm and less than orequal to 50 μm. One or both of the substrate 111 and the substrate 121may be a stacked-layer substrate that includes a plurality of layers.

It is preferable that the substrate 111 and the substrate 121 be formedusing the same material and have the same thickness. However, dependingon the purpose, the substrates 111 and 121 may be formed using differentmaterials or have different thicknesses.

Examples of materials that have flexibility and transmit visible light,which can be used for the substrate 111 and the substrate 121, include apolyethylene terephthalate resin, a polyethylene naphthalate resin, apolyacrylonitrile resin, a polyimide resin, a polymethylmethacrylateresin, a polycarbonate resin, a polyethersulfone resin, a polyamideresin, a cycloolefin resin, a polystyrene resin, a polyamide imideresin, a polyvinylchloride resin, and polytetrafluoroethylene (PTFE).Furthermore, when a light-transmitting property is not necessary, anon-light-transmitting substrate may be used. For example, aluminum orthe like may be used for the substrate 121 or the substrate 111.

The thermal expansion coefficients of the substrate 111 and thesubstrate 121 are preferably less than or equal to 30 ppm/K, morepreferably less than or equal to 10 ppm/K. In addition, on surfaces ofthe substrate 111 and the substrate 121, a protective film having lowwater permeability may be formed in advance; examples of the protectivefilm include a film containing nitrogen and silicon such as a siliconnitride film or a silicon oxynitride film and a film containing nitrogenand aluminum such as an aluminum nitride film. Note that a structure inwhich a fibrous body is impregnated with an organic resin (also calledprepreg) may be used as the substrate 111 and the substrate 121.

With such substrates, a non-breakable display device can be provided.Alternatively, a lightweight display device can be provided.Alternatively, an easily bendable display device can be provided.

For the layer 147, a material that is more flexible than the substrates111 and 121 is used. For example, a material having a smaller Young'smodulus than the substrate 111 is used for the layer 147.

The Young's modulus of the material used for the layer 147 is preferablysmaller than or equal to one fiftieth, further preferably smaller thanor equal to one hundredth, still further preferably smaller than orequal to one five hundredth of the Young's modulus of the materials usedfor the substrates 111 and 121.

Examples of a material that can be used for the layer 147 include aviscoelastic high molecular material such as silicone rubber or fluorinerubber. The material used for the layer 147 preferably has alight-transmitting property.

A material with a small Young's modulus more easily becomes deformedthan a material with a large Young's modulus does; therefore, internalstress generated by deformation is easily dispersed in the former. Whena material with a Young's modulus smaller than that of the substrate 111and the substrate 121 is used for the layer 147, local stress generatedin the substrate 111 and the substrate 121 at the time of bending can berelaxed, whereby the substrate 111 and the substrate 121 can beprevented from being broken. The layer 147 also functions as a bufferdispersing external physical pressure and impact.

The layer 147 can prevent the minimum radius of curvature of a bentportion from being smaller than the thickness of the layer 147.Therefore, breakage of the substrate 111 or the substrate 121 due tobending at an excessively small radius of curvature can be prevented.

In one embodiment of the present invention, the display device 100 canbe prevented from being broken even when the minimum curvature radius ofthe substrate 111 or 121 that is positioned on the inner side of a bentportion is 1 mm or less.

The thickness of the layer 147 is preferably greater than or equal to 2times and less than or equal to 100 times that of the substrate 111 andthe substrate 121, further preferably greater than or equal to 5 timesand less than or equal to 50 times that of the substrate 121. When thelayer 147 is thicker than the substrate 111 and the substrate 121,stress relaxation and the effect of buffers can be enhanced.

Depending on the usage of the display device, the layer 147 may have astacked structure formed of a plurality of layers.

In one embodiment of the present invention, a display device that isresistant to external impact and unlikely to be broken can be provided.

In one embodiment of the present invention, a highly reliable displaydevice can be provided which is unlikely to be broken even when it isrepeatedly bent and stretched.

The layer 147 that covers the edges (a side surfaces) of the substrate111 and the substrate 121 can prevent entry of impurity such as moisturefrom the edges. Therefore, the display device 100 can have highreliability and high display quality even when the frame of the displaydevice 100 is narrowed. In one embodiment of the present invention, theproductivity and design flexibility of the display device 100 can beimproved. Furthermore, the productivity and design flexibility of asemiconductor device including the display device of one embodiment ofthe present invention can be improved.

<Example of Pixel Circuit Configuration>

Next, an example of a specific configuration of the display device 100is described with reference to FIGS. 4A to 4C. FIG. 4A is a blockdiagram illustrating the configuration of the display device 100. Thedisplay device 100 includes the display region 131, the circuit 132, andthe circuit 133. The circuit 132 functions as a scan line drivercircuit, for example, and the circuit 133 functions as a signal linedriver circuit, for example.

The display device 100 includes m scan lines 135 which are arrangedparallel or substantially parallel to each other and whose potentialsare controlled by the circuit 132, and n signal lines 136 which arearranged parallel or substantially parallel to each other and whosepotentials are controlled by the circuit 133. The display region 131includes a plurality of pixels 130 arranged in a matrix.

Each of the scan lines 135 is electrically connected to the n pixels 130in the corresponding row among the pixels 130 arranged in m rows and ncolumns in the display region 131. Each of the signal lines 136 iselectrically connected to the m pixels 130 in the corresponding columnamong the pixels 130 arranged in m rows and n columns. Note that m and nare each an integer of 1 or more.

As illustrated in FIG. 5A, a circuit 152 may be provided on the oppositeside of the display region 131 from the circuit 132. Furthermore, asillustrated in FIG. 5B, a circuit 153 may be provided on the oppositeside of the display region 131 from the circuit 133. FIGS. 5A and 5Beach illustrate an example in which each scan line 135 is connected tothe circuit 152 and the circuit 132. However, the connection relation isnot limited to this. For example, each scan line 135 may be connected toone of the circuit 132 and the circuit 152. FIG. 5B illustrates anexample in which each signal line 136 is connected to the circuit 153and the circuit 133. However, the connection relation is not limited tothis. For example, each signal line 136 may be connected to one of thecircuit 133 and the circuit 153. The circuits 132, 133, 152, and 153 mayhave a function other than the function of driving the pixel 130.

In some cases, the circuits 132, 133, 152, and 153 may be collectivelycalled a driver circuit portion. The pixel 130 includes a pixel circuit137 and a display element. The pixel circuit 137 is a circuit thatdrives the display element. A transistor included in the driver circuitportion and a transistor included in the pixel circuit 137 can be formedat the same time. Part or the entire driver circuit portion may beformed over another substrate and electrically connected to the displaydevice 100. For example, part or the entire driver circuit portion maybe formed over a single crystal substrate and electrically connected tothe display device 100.

FIGS. 4B and 4C illustrate circuit configurations that can be used forthe pixels 130 in the display device illustrated in FIG. 4A.

[Example of Pixel Circuit for Light-Emitting Display Device]

The pixel circuit 137 illustrated in FIG. 4B includes a transistor 431,a capacitor 233, the transistor 232, and a transistor 434. The pixelcircuit 137 is electrically connected to the light-emitting element 125that can function as a display element.

One of a source electrode and a drain electrode of the transistor 431 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 431 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 431 has a function of controlling whether to write a datasignal to a node 435.

One of a pair of electrodes of the capacitor 233 is electricallyconnected to the node 435, and the other is electrically connected to anode 437. The other of the source electrode and the drain electrode ofthe transistor 431 is electrically connected to the node 435.

The capacitor 233 functions as a storage capacitor for storing datawritten to the node 435.

One of a source electrode and a drain electrode of the transistor 232 iselectrically connected to a potential supply line VL_a, and the other iselectrically connected to the node 437. A gate electrode of thetransistor 232 is electrically connected to the node 435.

One of a source electrode and a drain electrode of the transistor 434 iselectrically connected to a potential supply line V0, and the other ofthe source electrode and the drain electrode of the transistor 434 iselectrically connected to the node 437. A gate electrode of thetransistor 434 is electrically connected to the scan line GL_m.

One of an anode and a cathode of the light-emitting element 125 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the node 437.

As the light-emitting element 125, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 125 is not limited toorganic EL elements; an inorganic EL element including an inorganicmaterial can be used, for example.

As a power supply potential, a potential on the relatively highpotential side or a potential on the relatively low potential side canbe used, for example. A power supply potential on the high potentialside is referred to as a high power supply potential (also referred toas VDD), and a power supply potential on the low potential side isreferred to as a low power supply potential (also referred to as VSS). Aground potential can be used as the high power supply potential or thelow power supply potential. For example, in the case where a groundpotential is used as the high power supply potential, the low powersupply potential is a potential lower than the ground potential, and inthe case where a ground potential is used as the low power supplypotential, the high power supply potential is a potential higher thanthe ground potential.

For example, a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel circuit 137 in FIG. 4B, thepixel circuits 137 are sequentially selected row by row by the circuit132, whereby the transistors 431 and 434 are turned on and a data signalis written to the nodes 435.

When the transistors 431 and 434 are turned off, the pixel circuits 137in which the data has been written to the nodes 435 are brought into aholding state. Further, the amount of current flowing between the sourceelectrode and the drain electrode of the transistor 232 is controlled inaccordance with the potential of the data written to the node 435. Thelight-emitting element 125 emits light with a luminance corresponding tothe amount of flowing current. This operation is sequentially performedrow by row; thus, an image is displayed.

[Example of Pixel Circuit for Liquid Crystal Display Device]

The pixel circuit 137 illustrated in FIG. 4C includes the transistor 431and the capacitor 233. The pixel circuit 137 is electrically connectedto a liquid crystal element 432 that can function as a display element.

The potential of one of a pair of electrodes of the liquid crystalelement 432 is set according to the specifications of the pixel circuits137 as appropriate. The alignment state of the liquid crystal element432 depends on data written to a node 436. A common potential may beapplied to one of the pair of electrodes of the liquid crystal element432 included in each of the plurality of pixel circuits 137. Further,the potential supplied to one of a pair of electrodes of the liquidcrystal element 432 in the pixel circuit 137 in one row may be differentfrom the potential supplied to one of a pair of electrodes of the liquidcrystal element 432 in the pixel 137 in another row.

As examples of a driving method of the display device including theliquid crystal element 432, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optically compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude an electrically controlled birefringence (ECB) mode, a polymerdispersed liquid crystal (PDLC) mode, a polymer network liquid crystal(PNLC) mode, and a guest-host mode. Note that the present invention isnot limited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

The liquid crystal element 432 may be formed using a liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral material. The liquid crystal exhibiting a blue phase has a shortresponse time of 1 msec or less. Since the liquid crystal exhibiting ablue phase is optically isotropic, alignment treatment is not necessaryand viewing angle dependence is small.

In the pixel circuit 137 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 431 iselectrically connected to a signal line DL_n, and the other iselectrically connected to the node 436. A gate electrode of thetransistor 431 is electrically connected to a scan line GL_m. Thetransistor 431 has a function of controlling whether to write a datasignal to the node 436.

One of a pair of electrodes of the capacitor 233 is electricallyconnected to a wiring to which a particular potential is supplied(hereinafter referred to as a capacitor line CL), and the other iselectrically connected to the node 436. The other of the pair ofelectrodes of the liquid crystal element 432 is electrically connectedto the node 436. The potential of the capacitor line CL is set inaccordance with the specifications of the pixel circuit 137 asappropriate. The capacitor 233 functions as a storage capacitor forstoring data written to the node 436.

For example, in the display device including the pixel circuit 137 inFIG. 4C, the pixel circuits 137 are sequentially selected row by row bythe circuit 132, whereby the transistors 431 are turned on and a datasignal is written to the nodes 436.

When the transistors 431 are turned off, the pixel circuits 137 in whichthe data signal has been written to the nodes 436 are brought into aholding state. This operation is sequentially performed row by row;thus, an image is displayed on a display region 231.

[Display Element]

The display device of one embodiment of the present invention can employvarious modes and can include various display elements. Examples of thedisplay element include a display medium whose contrast, luminance,reflectance, transmittance, or the like is changed by electrical ormagnetic effect, such as an electroluminescence (EL) element (e.g., anEL element including organic and inorganic materials, an organic ELelement, or an inorganic EL element), an LED (e.g., a white LED, a redLED, a green LED, or a blue LED), a transistor (a transistor that emitslight depending on current), an electron emitter, a liquid crystalelement, electronic ink, an electrophoretic element, a grating lightvalve (GLV), a plasma display panel (PDP), a display element using microelectro mechanical system (MEMS), a digital micromirror device (DMD), adigital micro shutter (DMS), MIRASOL (registered trademark), aninterferometric modulator display (IMOD) element, a MEMS shutter displayelement, an optical-interference-type MEMS display element, anelectrowetting element, a piezoelectric ceramic display, or a displayelement using a carbon nanotube.

Alternatively, quantum dots may be used as the display element. Examplesof display devices having EL elements include an EL display. Examples ofa display device including an electron emitter include a field emissiondisplay (FED) and an SED-type flat panel display (SED:surface-conduction electron-emitter display). Examples of displaydevices including quantum dots include a quantum dot display. Examplesof display devices including liquid crystal elements include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of a display device including electronic ink,electronic liquid powder (registered trademark), or electrophoreticelements include electronic paper. In the case of a transflective liquidcrystal display or a reflective liquid crystal display, some or all ofpixel electrodes function as reflective electrodes. For example, some orall of pixel electrodes are formed to contain aluminum, silver, or thelike. In such a case, a memory circuit such as an SRAM can be providedunder the reflective electrodes, leading to lower power consumption.

Note that in the case of using an LED, graphene or graphite may beprovided under an electrode or a nitride semiconductor of the LED.Graphene or graphite may be a multilayer film in which a plurality oflayers are stacked. As described above, provision of graphene orgraphite enables easy formation of a nitride semiconductor filmthereover, such as an n-type GaN semiconductor layer including crystals.Furthermore, a p-type GaN semiconductor layer including crystals or thelike can be provided thereover, and thus the LED can be formed. Notethat an AlN layer may be provided between the n-type GaN semiconductorlayer including crystals and graphene or graphite. The GaN semiconductorlayers included in the LED may be formed by metal organic chemical vapordeposition (MOCVD). Note that when the graphene is provided, the GaNsemiconductor layers included in the LED can also be formed by asputtering method.

<Example of Pixel Configuration for Achieving Color Display>

Here, examples of a pixel configuration for achieving color display aredescribed with reference to FIGS. 6A and 6B. FIGS. 6A and 6B and FIGS.7A and 7B are enlarged plan views of a region 170 in the display region131 of FIG. 1. As illustrated in FIG. 6A, for example, each pixel 130may function as a subpixel and three pixels 130 may be collectively usedas one pixel 140. The use of a red, a green, and a blue coloring layersas the coloring layers 266 for the three pixels 130 enables full-colordisplay. In FIG. 6A, the pixel 130 emitting red light, the pixel 130emitting green light, and the pixel 130 emitting blue light areillustrated as a pixel 130R, a pixel 130G, and a pixel 130B,respectively. The colors of the coloring layers 266 may be a color otherthan red, green, and blue; for example, the colors of the coloring layer266 may be yellow, cyan, magenta, or the like.

As illustrated in FIG. 6B, each pixel 130 may function as a subpixel andfour pixels 130 may be collectively used as one pixel 140. For example,the coloring layers 266 corresponding to the four pixels 130 may be red,green, blue, and yellow. In FIG. 6B, the pixel 130 emitting red light,the pixel 130 emitting green light, the pixel 130 emitting blue light,and the pixel 130 emitting yellow light are illustrated as a pixel 130R,a pixel 130G, a pixel 130B, and a pixel 130Y, respectively. Byincreasing the number of subpixels (pixels 130) included in one pixel140, the color reproducibility can be particularly improved.

Alternatively, the coloring layers 266 corresponding to the four pixels130 may be red, green, blue, and white (see FIG. 6B). With the pixel 130emitting white light (pixel 130W), the luminance of the display regioncan be increased. Note that in the case where the pixel 130W emittingwhite light is provided, it is not necessary to provide the coloringlayer 266 for the pixel 130W. Without the coloring layer 266 for thepixel 130W, there is no luminance reduction at the time of transmittinglight through the coloring layer 266; thus, the luminance of the displayregion can be increased. Moreover, power consumption of the displaydevice can be reduced. On the other hand, color temperature of whitelight can be controlled with the coloring layer 266 for the pixel 130W.Thus, the display quality of the display device can be improved.Depending on the intended use of the display device, each pixel 130 mayfunction as a subpixel and two pixels 130 may be collectively used asone pixel 140.

In the case where the four pixels 130 are collectively used as one pixel140, the four pixels 130 may be arranged in a matrix, as in FIG. 7B. Inaddition, in the case where the four pixels 130 are collectively used asone pixel 140, a pixel that emits light of cyan, magenta, or the likemay be used instead of the pixel 130Y or the pixel 130W. A plurality ofpixels 130 that emit light of the same color may be provided in thepixel 140.

Note that the occupation areas or shapes of the pixels 130 included inthe pixel 140 may be the same or different. In addition, arrangement isnot limited to stripe arrangement or matrix arrangement. For example,delta arrangement, Bayer arrangement, pentile arrangement, or the likecan be used. FIG. 7A illustrates an example of pentile arrangement.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 2

In this embodiment, an example of a method for manufacturing the displaydevice 100 is described with reference to FIGS. 8A to 8D, FIGS. 9A to9D, FIGS. 10A to 10D, FIGS. 11A and 11B, FIGS. 12A and 12B, FIGS. 13Aand 13B, FIGS. 14A and 14B, FIGS. 15A and 15B, FIGS. 16A to 16C, FIG.17, FIGS. 18A1 to 18B3, FIG. 19, and FIGS. 20A and 20B. Note that FIGS.8A to 8D, FIGS. 9A to 9D, FIGS. 11A and 11B, FIGS. 12A and 12B, FIGS.13A and 13B, FIGS. 14A and 14B, and FIGS. 20A and 20B correspond to thecross section taken along the dashed-dotted line A1-A2 in FIG. 1.

<Example of Method for Manufacturing Display Device>

[Formation of Separation Layer]

First, a separation layer 113 is formed over a substrate 101 (see FIG.8A). As the substrate 101, a glass substrate, a quartz substrate, asapphire substrate, a ceramic substrate, a metal substrate, or the likecan be used. Alternatively, a plastic substrate having heat resistanceto the processing temperature in this embodiment may be used.

As the glass substrate, for example, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used. Note that when the glass substrate contains a largeamount of barium oxide (BaO), the glass substrate can be heat-resistantand more practical. Alternatively, crystallized glass or the like can beused.

The separation layer 113 can be formed using an element selected fromtungsten, molybdenum, titanium, tantalum, niobium, nickel, cobalt,zirconium, ruthenium, rhodium, palladium, osmium, iridium, and silicon;an alloy material containing any of the elements; or a compound materialcontaining any of the elements.

The separation layer 113 can also be formed to have a single-layerstructure or a stacked-layer structure using any of the materials. Notethat the crystalline structure of the separation layer 113 may beamorphous, microcrystalline, or polycrystalline. The separation layer113 can also be formed using a metal oxide such as aluminum oxide,gallium oxide, zinc oxide, titanium dioxide, indium oxide, indium tinoxide, indium zinc oxide, or InGaZnO (IGZO).

The separation layer 113 can be formed by a sputtering method, a CVDmethod, a coating method, a printing method, or the like. Note that thecoating method includes a spin coating method, a droplet dischargemethod, and a dispensing method.

In the case where the separation layer 113 has a single-layer structure,a material containing tungsten, a material containing molybdenum, or amaterial containing tungsten and molybdenum is preferably used.Alternatively, in the case where the separation layer 113 has asingle-layer structure, an oxide or oxynitride of tungsten, an oxide oroxynitride of molybdenum, or an oxide or oxynitride of a materialcontaining tungsten and molybdenum is preferably used.

In the case where the separation layer 113 has a stacked-layer structureincluding, for example, a layer containing tungsten and a layercontaining an oxide of tungsten, the layer containing an oxide oftungsten may be formed as follows: the layer containing tungsten isformed first and then an oxide insulating layer is formed in contacttherewith, so that the layer containing an oxide of tungsten is formedat the interface between the layer containing tungsten and the oxideinsulating layer. Alternatively, the layer containing an oxide oftungsten may be formed by performing thermal oxidation treatment, oxygenplasma treatment, treatment with a highly oxidizing solution such asozone water, or the like on the surface of the layer containingtungsten.

In this embodiment, a glass substrate is used as the substrate 101. Asthe separation layer 113, a tungsten layer is formed over the substrate101 by a sputtering method.

[Formation of Insulating Layer]

Next, the insulating layer 205 is formed as a base layer over theseparation layer 113 (see FIG. 8A). The insulating layer 205 ispreferably formed as a single layer or a multilayer using any of siliconoxide, silicon nitride, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum oxynitride, and aluminum nitride oxide. Theinsulating layer 205 may have, for example, a two-layer structure ofsilicon oxide and silicon nitride or a five-layer structure in whichmaterials selected from the above are combined. The insulating layer 205can be formed by a sputtering method, a CVD method, a thermal oxidationmethod, a coating method, a printing method, or the like.

The thickness of the insulating layer 205 is greater than or equal to 30nm and less than or equal to 500 nm, preferably greater than or equal to50 nm and less than or equal to 400 nm.

The insulating layer 205 can prevent or reduce diffusion of impurityelements from the substrate 101, the separation layer 113, or the like.Even after the substrate 101 is replaced with the substrate 111, theinsulating layer 205 can prevent or reduce diffusion of impurityelements into the light-emitting element 125 from the substrate 111, thebonding layer 112, or the like. In this embodiment, the insulating layer205 is formed by stacking a 200-nm-thick silicon oxynitride film and a50-nm-thick silicon nitride oxide film by a plasma CVD method.

[Formation of Gate Electrode]

Next, the gate electrode 206 is formed over the insulating layer 205(see FIG. 8A). The gate electrode 206 can be formed using a metalelement selected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing any of these metalelements as a component; an alloy containing any of these metal elementsin combination; or the like. Further, one or more metal elementsselected from manganese and zirconium may be used. The gate electrode206 may have a single-layer structure or a stacked structure of two ormore layers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which an aluminum film isstacked over a titanium film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a two-layer structure in whicha copper film is stacked over a titanium film, a three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order, and the like can be given. Alternatively, a film,an alloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 206 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

First, a conductive film to be the gate electrode 206 is stacked overthe insulating layer 205 by a sputtering method, a CVD method, anevaporation method, or the like, and a resist mask is formed over theconductive film by a photolithography process. Next, part of theconductive film to be the gate electrode 206 is etched with the use ofthe resist mask to form the gate electrode 206. At the same time, awiring and another electrode can be formed.

The conductive film may be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Note thatin the case where the conductive film is etched by a dry etching method,ashing treatment may be performed before the resist mask is removed,whereby the resist mask can be easily removed using a stripper.

Note that the gate electrode 206 may be formed by an electrolyticplating method, a printing method, an inkjet method, or the like insteadof the above formation method.

The thickness of the gate electrode 206 is greater than or equal to 5 nmand less than or equal to 500 nm, preferably greater than or equal to 10nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm.

The gate electrode 206 may be formed using a light-blocking conductivematerial, whereby external light can be prevented from reaching thesemiconductor layer 208 from the gate electrode 206 side. As a result, avariation in electrical characteristics of the transistor due to lightirradiation can be suppressed.

[Formation of Gate Insulating Layer]

Next, the gate insulating layer 207 is formed (see FIG. 8A). The gateinsulating layer 207 can be formed to have a single-layer structure or astacked-layer structure using, for example, any of silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, a mixture of aluminum oxide and silicon oxide, hafnium oxide,gallium oxide, Ga—Zn-based metal oxide, and the like.

The gate insulating layer 207 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced. For example, a stackedlayer of silicon oxynitride and hafnium oxide may be used.

The thickness of the gate insulating layer 207 is preferably greaterthan or equal to 5 nm and less than or equal to 400 nm, furtherpreferably greater than or equal to 10 nm and less than or equal to 300nm, still further preferably greater than or equal to 50 nm and lessthan or equal to 250 nm.

The gate insulating layer 207 can be formed by a sputtering method, aCVD method, an evaporation method, or the like.

In the case where a silicon oxide film, a silicon oxynitride film, or asilicon nitride oxide film is formed as the gate insulating layer 207, adeposition gas containing silicon and an oxidizing gas are preferablyused as a source gas. Typical examples of the deposition gas containingsilicon include silane, disilane, trisilane, and silane fluoride. As theoxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, andthe like can be given as examples.

The gate insulating layer 207 can have a stacked-layer structure inwhich a nitride insulating layer and an oxide insulating layer arestacked in this order from the gate electrode 206 side. When the nitrideinsulating layer is provided on the gate electrode 206 side, hydrogen,nitrogen, an alkali metal, an alkaline earth metal, or the like can beprevented from moving from the gate electrode 206 side to thesemiconductor layer 208. Note that nitrogen, an alkali metal, analkaline earth metal, or the like generally serves as an impurityelement of a semiconductor. In addition, hydrogen serves as an impurityelement of an oxide semiconductor. Thus, an “impurity” in thisspecification and the like includes hydrogen, nitrogen, an alkali metal,an alkaline earth metal, or the like.

In the case where an oxide semiconductor is used for the semiconductorlayer 208, the density of defect states at the interface between thegate insulating layer 207 and the semiconductor layer 208 can be reducedby providing the oxide insulating layer on the semiconductor layer 208side. Consequently, a transistor whose electrical characteristics arehardly degraded can be obtained. Note that in the case where an oxidesemiconductor is used for the semiconductor layer 208, an oxideinsulating layer containing oxygen in a proportion higher than that inthe stoichiometric composition is preferably formed as the oxideinsulating layer. This is because the density of defect states at theinterface between the gate insulating layer 207 and the semiconductorlayer 208 can be further reduced.

In the case where the gate insulating layer 207 is a stacked layer of anitride insulating layer and an oxide insulating layer as describedabove, it is preferable that the nitride insulating layer be thickerthan the oxide insulating layer.

The nitride insulating layer has a dielectric constant higher than thatof the oxide insulating layer; therefore, an electric field generatedfrom the gate electrode 206 can be efficiently transmitted to thesemiconductor layer 208 even when the gate insulating layer 207 has alarge thickness. When the gate insulating layer 207 has a large totalthickness, the withstand voltage of the gate insulating layer 207 can beincreased. Accordingly, the reliability of the semiconductor device canbe improved.

The gate insulating layer 207 can have a stacked-layer structure inwhich a first nitride insulating layer with few defects, a secondnitride insulating layer with a high blocking property against hydrogen,and an oxide insulating layer are stacked in that order from the gateelectrode 206 side. When the first nitride insulating layer with fewdefects is used in the gate insulating layer 207, the withstand voltageof the gate insulating layer 207 can be improved. Particularly when anoxide semiconductor is used for the semiconductor layer 208, the use ofthe second nitride insulating layer with a high blocking propertyagainst hydrogen in the gate insulating layer 207 makes it possible toprevent hydrogen contained in the gate electrode 206 and the firstnitride insulating layer from moving to the semiconductor layer 208.

An example of a method for forming the first and second nitrideinsulating layers is described below. First, a silicon nitride film withfew defects is formed as the first nitride insulating layer by a plasmaCVD method in which a mixed gas of silane, nitrogen, and ammonia is usedas a source gas. Next, a silicon nitride film in which the hydrogenconcentration is low and hydrogen can be blocked is formed as the secondnitride insulating layer by switching the source gas to a mixed gas ofsilane and nitrogen. By such a formation method, the gate insulatinglayer 207 in which nitride insulating layers with few defects and ablocking property against hydrogen are stacked can be formed.

The gate insulating layer 207 can have a structure in which a thirdnitride insulating layer with a high blocking property against animpurity, the first nitride insulating layer with few defects, thesecond nitride insulating layer with a high blocking property againsthydrogen, and the oxide insulating layer are stacked in that order fromthe gate electrode 206 side. When the third nitride insulating layerwith a high blocking property against an impurity is provided in thegate insulating layer 207, hydrogen, nitrogen, alkali metal, alkalineearth metal, or the like, can be prevented from moving from the gateelectrode 206 to the semiconductor layer 208.

An example of a method for forming the first to third nitride insulatinglayers is described below. First, a silicon nitride film with a highblocking property against an impurity is formed as the third nitrideinsulating layer by a plasma CVD method in which a mixed gas of silane,nitrogen, and ammonia is used as a source gas. Next, a silicon nitridefilm with few defects is formed as the first nitride insulating layer byincreasing the flow rate of ammonia. Then, a silicon nitride film inwhich the hydrogen concentration is low and hydrogen can be blocked isformed as the second nitride insulating layer by switching the sourcegas to a mixed gas of silane and nitrogen. By such a formation method,the gate insulating layer 207 in which nitride insulating layers withfew defects and a blocking property against an impurity are stacked canbe formed.

Moreover, in the case of forming a gallium oxide film as the gateinsulating layer 207, an MOCVD method can be employed.

Note that the threshold voltage of a transistor can be changed bystacking the semiconductor layer 208 in which a channel of thetransistor is formed and an insulating layer containing hafnium oxidewith an oxide insulating layer provided therebetween and injectingelectrons into the insulating layer containing hafnium oxide.

[Formation of Semiconductor Layer]

The semiconductor layer 208 can be formed using an amorphoussemiconductor, a microcrystalline semiconductor, a polycrystallinesemiconductor, or the like. For example, amorphous silicon ormicrocrystalline germanium can be used. Alternatively, a compoundsemiconductor such as silicon carbide, gallium arsenide, an oxidesemiconductor, or a nitride semiconductor, an organic semiconductor, orthe like can be used.

First, a semiconductor film for forming the semiconductor layer 208 isformed by a CVD method such as a plasma CVD method, an LPCVD method, ametal CVD method, or an MOCVD method, an ALD method, a sputteringmethod, an evaporation method, or the like. When the semiconductor filmis formed by an MOCVD method, damage to a surface where thesemiconductor layer is formed can be reduced.

Next, a resist mask is formed over the semiconductor film, and part ofthe semiconductor film is selectively etched using the resist mask toform the semiconductor layer 208. The resist mask can be formed by aphotolithography method, a printing method, an inkjet method, or thelike as appropriate. Formation of the resist mask by an inkjet methodneeds no photomask; thus, fabrication cost can be reduced.

Note that the etching of the semiconductor film may be performed byeither one or both of a dry etching method and a wet etching method.After the etching of the semiconductor film, the resist mask is removed(see FIG. 8B).

[Formation of Source Electrode, Drain Electrode, and the Like]

Next, the source electrode 209 a, the drain electrode 209 b, the wiring219, and the terminal electrode 216 are formed (see FIG. 8C). First, aconductive film, which forms the source electrode 209 a, the drainelectrode 209 b, the wiring 219, and the terminal electrode 216, isformed over the gate insulating layer 207 and the semiconductor layer208.

The conductive film can have a single-layer structure or a stacked-layerstructure containing any of metals such as aluminum, titanium, chromium,nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, andtungsten or an alloy containing any of these metals as its maincomponent. For example, the following structures can be given: asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which an aluminum film is stacked over a titaniumfilm, a two-layer structure in which an aluminum film is stacked over atungsten film, a two-layer structure in which a copper film is stackedover a copper-magnesium-aluminum alloy film, a two-layer structure inwhich a copper film is stacked over a titanium film, a two-layerstructure in which a copper film is stacked over a tungsten film, athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order, a three-layer structurein which a molybdenum film or a molybdenum nitride film, an aluminumfilm or a copper film, and a molybdenum film or a molybdenum nitridefilm are stacked in this order, and a three-layer structure in which atungsten film, a copper film, and a tungsten film are stacked in thisorder.

Note that a conductive material containing oxygen such as indium tinoxide, zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, or a conductive materialcontaining nitrogen such as titanium nitride or tantalum nitride may beused. It is also possible to use a stacked-layer structure formed usinga material containing the above metal element and conductive materialcontaining oxygen. It is also possible to use a stacked-layer structureformed using a material containing the above metal element and aconductive material containing nitrogen. It is also possible to use astacked-layer structure formed using a material containing the abovemetal element, a conductive material containing oxygen, and a conductivematerial containing nitrogen.

The thickness of the conductive film is greater than or equal to 5 nmand less than or equal to 500 nm, preferably greater than or equal to 10nm and less than or equal to 300 nm, more preferably greater than orequal to 10 nm and less than or equal to 200 nm. In this embodiment, a300-nm-thick tungsten film is formed as the conductive film.

Then, part of the conductive film is selectively etched using a resistmask to form the source electrode 209 a, the drain electrode 209 b, thewiring 219, and the terminal electrode 216 (including other electrodesand wirings formed using the same layer). The resist mask can be formedby a photolithography method, a printing method, an inkjet method, orthe like as appropriate. Formation of the resist mask by an inkjetmethod needs no photomask; thus, fabrication cost can be reduced.

The conductive film may be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Note thatan exposed portion of the semiconductor layer 208 is removed by theetching step in some cases. After the etching of the conductive film,the resist mask is removed.

With the source electrode 209 a and the drain electrode 209 b, thetransistor 232 and the transistor 252 are completed.

[Formation of Insulating Layer]

Next, the insulating layer 210 is formed over the source electrode 209a, the drain electrode 209 b, the wiring 219, and the terminal electrode216 (see FIG. 8D). The insulating layer 210 can be formed using amaterial and a method similar to those of the insulating layer 205.

In the case where an oxide semiconductor is used for the semiconductorlayer 208, an insulating layer containing oxygen is preferably used forat least part of the insulating layer 210 that is in contact with thesemiconductor layer 208. For example, in the case where the insulatinglayer 210 is a stack of a plurality of layers, at least a layer that isin contact with the semiconductor layer 208 is preferably formed usingsilicon oxide.

[Formation of Opening]

Next, part of the insulating layer 210 is selectively etched using aresist mask to form an opening 128 (see FIG. 8D). At the same time,another opening that is not illustrated can also be formed. The resistmask can be formed by a photolithography method, a printing method, aninkjet method, or the like as appropriate. Formation of the resist maskby an inkjet method needs no photomask; thus, fabrication cost can bereduced.

The insulating layer 210 may be etched by a dry etching method, a wetetching method, or both a dry etching method and a wet etching method.

The drain electrode 209 b and the terminal electrode 216 are partlyexposed by the formation of the opening 128. The resist mask is removedafter the formation of the opening 128.

[Formation of Planarization Film]

Next, the insulating layer 211 is formed over the insulating layer 210(see FIG. 9A). The insulating layer 211 can be formed using a materialand a method similar to those of the insulating layer 205.

Planarization treatment may be performed on the insulating layer 211 toreduce unevenness of a surface on which the light-emitting element 125is formed. The planarization treatment may be, but not particularlylimited to, polishing treatment (e.g., chemical mechanical polishing(CMP)) or dry etching treatment.

Forming the insulating layer 211 using an insulating material with aplanarization function can make polishing treatment unnecessary. As theinsulating material with a planarization function, for example, anorganic material such as a polyimide resin or an acrylic resin can beused. Besides such organic materials, a low-dielectric constant material(a low-k material) or the like can be used. Note that the insulatinglayer 211 may be formed by stacking a plurality of insulating layersformed of any of these materials.

Part of the insulating layer 211 that overlaps with the opening 128 isremoved to form an opening 129. At the same time, another opening thatis not illustrated is also formed. In addition, the insulating layer 211in a region to which the external electrode 124 is connected later isremoved. Note that the opening 129 or the like can be formed in such amanner that a resist mask is formed by a photolithography process overthe insulating layer 211 and a region of the insulating layer 211 thatis not covered with the resist mask is etched. A surface of the drainelectrode 209 b is exposed by the formation of the opening 129.

When the insulating layer 211 is formed using a photosensitive material,the opening 129 can be formed without the resist mask. In thisembodiment, a photosensitive polyimide resin is used to form theinsulating layer 211 and the opening 129.

[Formation of Anode]

Next, the electrode 115 is formed over the insulating layer 211 (seeFIG. 9B). The electrode 115 is preferably formed using a conductivematerial that efficiently reflects light emitted from the EL layer 117formed later. Note that the electrode 115 may have a stacked-layerstructure of a plurality of layers without limitation to a single-layerstructure. For example, in the case where the electrode 115 is used asan anode, a layer in contact with the EL layer 117 may be alight-transmitting layer, such as an indium tin oxide layer, having awork function higher than that of the EL layer 117, and a layer havinghigh reflectance (e.g., aluminum, an alloy containing aluminum, orsilver) may be provided in contact with the layer.

The electrode 115 can be formed in such a manner that a conductive filmto be the electrode 115 is formed over the insulating layer 211, aresist mask is formed over the conductive film, and a region of theconductive film that is not covered with the resist mask is etched. Theconductive film can be etched by a dry etching method, a wet etchingmethod, or both a dry etching method and a wet etching method. Theresist mask can be formed by a photolithography method, a printingmethod, an inkjet method, or the like as appropriate. Formation of theresist mask by an inkjet method needs no photomask; thus, fabricationcost can be reduced. The resist mask is removed after the formation ofthe electrode 115.

[Formation of Partition]

Next, the partition 114 is formed (see FIG. 9C). The partition 114 isprovided in order to prevent an unintended electrical short-circuitbetween light-emitting elements 125 in adjacent pixels and unintendedlight emission from the light-emitting element 125. In the case of usinga metal mask for formation of the EL layer 117 described later, thepartition 114 has a function of preventing the contact of the metal maskwith the electrode 115. The partition 114 can be formed of an organicresin material such as an epoxy resin, an acrylic resin, or an imideresin or an inorganic material such as silicon oxide. The partition 114is preferably formed so that its sidewall has a tapered shape or atilted surface with a continuous curvature. The sidewall of thepartition 114 having the above-described shape enables favorablecoverage with the EL layer 117 and the electrode 118 formed later.

[Formation of EL Layer]

A structure of the EL layer 117 is described in Embodiment 6.

[Formation of Cathode]

The electrode 118 is used as a cathode in this embodiment, and thus ispreferably formed using a material that has a low work function and caninject electrons into the EL layer 117 described later. As well as asingle-layer of a metal having a low work function, a stack in which ametal material such as aluminum, a conductive oxide material such asindium tin oxide, or a semiconductor material is formed over aseveral-nanometer-thick buffer layer formed of an alkali metal or analkaline earth metal having a low work function may be used as theelectrode 118.

In the case where light emitted from the EL layer 117 is extractedthrough the electrode 118, the electrode 118 preferably has a propertyof transmitting visible light. The light-emitting element 125 includesthe electrode 115, the EL layer 117, and the electrode 118 (see FIG.9D).

In this embodiment, the substrate 101 including the transistor 232 andthe light-emitting element 125 is referred to as an element substrate171.

[Formation of Counter Substrate]

The separation layer 143 and the insulating layer 145 are formed overthe element formation substrate 141 (see FIG. 10A). The elementformation substrate 141 can be formed using a material similar to thatof the substrate 101. The separation layer 143 can be formed using amaterial and a method similar to those of the separation layer 113. Theinsulating layer 145 can be formed using a material and a method similarto those of the insulating layer 205.

Next, the light-blocking layer 264 is formed over the insulating layer145 (see FIG. 10B). After that, the coloring layer 266 is formed (seeFIG. 10C).

The light-blocking layer 264 and the coloring layer 266 each are formedin a desired position with any of various materials by a printingmethod, an inkjet method, a photolithography method, or the like.

Next, the overcoat layer 268 is formed over the light-blocking layer 264and the coloring layer 266 (see FIG. 10D).

For the overcoat layer 268, an organic insulating layer of an acrylicresin, an epoxy resin, polyimide, or the like can be used. With theovercoat layer 268, for example, an impurity or the like contained inthe coloring layer 266 can be prevented from diffusing into thelight-emitting element 125 side. Note that the overcoat layer 268 is notnecessarily formed.

A light-transmitting conductive film may be formed as the overcoat layer268. The light-transmitting conductive film is formed as the overcoatlayer 268, so that the light 235 emitted from the light-emitting element125 can be transmitted through the overcoat layer 268 and layersoverlapping with the overcoat layer 268, while ionized impurities can beprevented from passing through the overcoat layer 268.

The light-transmitting conductive film can be formed using, for example,indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide, orzinc oxide to which gallium is added. Graphene or a metal film that isthin enough to have a light-transmitting property can also be used.

Through the above-described steps, the components such as the coloringlayer 266 can be formed over the element formation substrate 141. Inthis embodiment, the element formation substrate 141 including thecoloring layer 266 and the like is referred to as a counter substrate181.

[Attachment of Element Substrate to Counter Substrate]

Next, the element substrate 171 is attached to the counter substrate 181with bonding layer 120 positioned therebetween such that thelight-emitting element 125 included in the element substrate 171 facesthe coloring layer 266 included in the counter substrate 181 (see FIG.11A).

A light curable adhesive, a reactive curable adhesive, a thermosettingadhesive, or an anaerobic adhesive can be used as the bonding layer 120.For example, an epoxy resin, an acrylic resin, or an imide resin can beused. In a top-emission structure, a drying agent (e.g., zeolite) havinga size less than or equal to the wavelength of light or a filler (e.g.,titanium oxide or zirconium) with a high refractive index is preferablymixed into the bonding layer 120, in which case the efficiency ofextracting light emitted from the EL layer 117 can be improved.

[Separation of Substrate 101 from Insulating Layer]

Next, the substrate 101 and the separation layer 113 are separated fromthe insulating layer 205 (see FIG. 11B). As a separation method,mechanical force (a separation process with a human hand or a gripper, aseparation process by rotation of a roller, ultrasonic waves, or thelike) may be used. For example, a cut is made in the separation layer113 with a sharp edged tool, by laser light irradiation, or the like andwater is injected into the cut. Alternatively, the cut is sprayed with amist of water. A portion between the separation layer 113 and theinsulating layer 205 absorbs water through capillarity action, so thatthe substrate 101 with the separation layer 113 can be separated easilyfrom the insulating layer 205.

[Bonding of Substrate 111]

Next, the substrate 111 is attached to the insulating layer 205 with thebonding layer 112 therebetween (see FIGS. 12A and 12B). The bondinglayer 112 can be formed using a material similar to that of the bondinglayer 120. In this embodiment, a 20-μm-thick aramid (polyamide resin)with a Young's modulus of 10 GPa is used for the substrate 111.

[Separation of Element Formation Substrate 141 from Insulating Layer]

Next, the element formation substrate 141 with the separation layer 143is separated from the insulating layer 145 (see FIG. 13A). The elementformation substrate 141 can be separated in a manner similar to that ofthe above-described separation method of the substrate 101.

[Bonding of Substrate 121]

Next, the substrate 121 is attached to the insulating layer 145 with thebonding layer 142 therebetween (see FIG. 13B). The bonding layer 142 canbe formed using a material similar to that of the bonding layer 120. Thesubstrate 121 can be formed using a material similar to that of thesubstrate 111.

[Formation of Opening]

Next, the substrate 121, the bonding layer 142, the insulating layer145, the coloring layer 266, the overcoat layer 268, and the bondinglayer 120 in a region overlapping with the terminal electrode 216 andthe opening 128 are removed to form the opening 122 (see FIG. 14A). Asurface of the terminal electrode 216 is partly exposed by the formationof the opening 122.

[Formation of External Electrode]

Next, the anisotropic conductive connection layer 123 is formed in andover the opening 122, and the external electrode 124 is formed over theanisotropic conductive connection layer 123 (see FIG. 14B). The externalelectrode 124 is electrically connected to the terminal electrode 216through the anisotropic conductive connection layer 123. Power or asignal is supplied to the display device 100 through the externalelectrode 124 and the terminal electrode 216. For example, a flexibleprinted circuit (FPC) or a tape carrier package (TCP) can be used as theexternal electrode 124. The TCP is, for example, a tape automatedbonding (TAB) tape mounted with a semiconductor chip on which anintegrated circuit is formed. The semiconductor chip is electricallyconnected to the terminal electrode 216 through the TAB tape.

The anisotropic conductive connection layer 123 can be formed using anyof various anisotropic conductive films (ACF), anisotropic conductivepastes (ACP), and the like.

The anisotropic conductive connection layer 123 is formed by curing apaste-form or sheet-form material that is obtained by mixing conductiveparticles to a thermosetting resin or a thermosetting, light curableresin. The anisotropic conductive connection layer 123 exhibits ananisotropic conductive property by light irradiation orthermocompression bonding. As the conductive particles used for theanisotropic conductive connection layer 123, for example, particles of aspherical organic resin coated with a thin-film metal such as Au, Ni, orCo can be used.

Note that a metal wire can also be used as the external electrode 124.Although the anisotropic conductive connection layer 123 may be used toconnect the metal wire and the terminal electrode 216 to each other, theconnection may be performed by a wire bonding method without using theanisotropic conductive connection layer 123. Alternatively, the metalwire and the terminal electrode 216 may be connected to each other by asoldering method.

In the above-described manner, the display device 100 to which theexternal electrode 124 is connected can be fabricated. FIG. 15A is aperspective view of the display device 100 to which the externalelectrode 124 is connected. Note that the substrate 121 may be formed tocover the display region 131, the circuit 132, and the circuit 133 andnot to cover the other regions. An example of a display device havingsuch a structure is illustrated in FIG. 15B. A display device 200illustrated in FIG. 15B is different from the display device 100 in thatthe substrate 121 is not provided in a connection region of the externalelectrode 124. Therefore, the external shapes of the substrate 111 andthe substrate 121 included in the display device 200 are different.

[Formation of Layer 147]

Next, the display device 100 is covered with the layer 147. An exampleof a method for forming the layer 147 that covers the display device 100is described with reference to FIGS. 16A to 16C. A metallic mold 191illustrated in FIG. 16A has a depressed portion 192. A metallic mold 193has a depressed portion 194. The depressed portion 192 and the depressedportion 194 are preferably similar in shape. The surfaces of thedepressed portion 192 and the depressed portion 194 preferably have highplanarity by being subjected to mirror finishing or the like.

First, the metallic mold 191 and the metallic mold 193 are overlappedsuch that the depressed portion 192 and the depressed portion 194 faceeach other. Next, the display device 100 to which the external electrode124 is connected is disposed in a space surrounded by the depressedportion 192 and the depressed portion 194 (see FIG. 16B).

Next, the space surrounded by the depressed portion 192 and thedepressed portion 194 is filled with a liquid filler 195 (see FIG. 16C).As the filler 195, it is preferable to use a high molecular materialthat exhibits a light transmitting property after being cured. As thefiller 195, a single-component-type material that does not need a curingagent or a two-component-type material that is cured by being mixed witha curing agent can be used, for example. Alternatively, a material thatis cured by heating, irradiation with light such as ultraviolet lightcan be used. A desiccant that inhibits permeation of moisture may beadded to the filler 195.

In this embodiment, a two-component-type material that becomeslight-transmitting silicone rubber after being cured is used as thefiller 195.

The filler 195 is cured to have the shape of the depressed portion 192and the depressed portion 194, whereby the layer 147 can be formed.After the formation of the layer 147, the metallic mold 191 and themetallic mold 193 are separated (see FIG. 17). Note that it ispreferable to apply a remover onto surfaces of the depressed portion 192and the depressed portion 194 before the space is filled with the filler195, in which case the layer 147 can be separated easily from themetallic mold 191 and the metallic mold 193.

FIG. 18A1 is a perspective view of the display device 100 to which theexternal electrode 124 is connected and which is covered with the layer147. FIG. 18A2 is a cross-sectional view taken along the dashed-dottedline H1-H2 in FIG. 18A1. With the layer 147 that covers the displaydevice 100, the display device is less likely to be broken even whenbeing bent and extended repeatedly. The layer 147 that covers thedisplay device 100 is seamless. By covering the edges of the substrate111 and the substrate 121 with the layer 147, entry of impurity such asmoisture from the edges can be prevented, whereby the display device 100can have high reliability and high display quality.

FIG. 18B1 is a perspective view of the display device 200 to which theexternal electrode 124 is connected and which is covered with the layer147. FIG. 18B2 is a cross-sectional view taken along the dashed-dottedline H1-H2 in FIG. 18A1. In the case where the substrate 111 and thesubstrate 121 have different external dimensions as in the displaydevice 200, edges (side surfaces) of one of the substrate 111 and thesubstrate 121 may be covered with the layer 147. By covering the sidesurfaces of at least one of the substrate 111 and the substrate 121 withthe layer 147, an outer periphery of a portion where the substrate 111and the substrate 121 overlap each other is covered with the layer 147.Therefore, impurity such as moisture can be prevented from entering thedisplay region 131, whereby the display device 200 can have highreliability and high display quality. Alternatively, as illustrated inFIG. 18B3, the layer 147 may be provided to cover both of the substrates111 and 121 having different external dimensions. FIG. 19 is a detailedcross-sectional view taken along the dashed-dotted line H5-H6 in FIG.18B1.

<Modification Example of Display Device>

FIG. 20A is a cross-sectional view of the display device 100 including atouch sensor 270 between the substrate 121 and the coloring layer 266.Specifically, the display device 100 illustrated in FIG. 20A includes anelectrode 272, an insulating layer 273, an electrode 274, and aninsulating layer 275 between the insulating layer 145 and the coloringlayer 266. The electrodes 272 and 274 are preferably formed with alight-transmitting conductive material. The insulating layer 273 and theinsulating layer 275 can be formed using a material and a method similarto those of the insulating layer 205. The touch sensor 270 includes theelectrode 272 and the electrode 274. Although an example in which thetouch sensor 270 is a capacitance touch sensor is described in thisembodiment, the touch sensor 270 may be a resistive touch sensor.Examples of the capacitive touch sensor are of a surface capacitive typeand of a projected capacitive type. Alternatively, an active matrixtouch sensor using an active element such as a transistor can be used.

Note that a low resistance material is preferably used for a conductivefilm such as the electrodes 272 and 274, i.e., a wiring or an electrode,included in the touch sensor. For example, silver, copper, aluminum, acarbon nanotube, graphene, or a metal halide (such as a silver halide)may be used. Alternatively, a metal nanowire including a number ofconductors with an extremely small width (for example, a diameter ofseveral nanometers) may be used. Further alternatively, a net-like metalmesh with a conductor may be used. Examples of such materials include anAg nanowire, a Cu nanowire, an Al nanowire, an Ag mesh, a Cu mesh, andan Al mesh. In the case of using an Ag nanowire, a light transmittanceof 89% or more and a sheet resistance of 40 ohm/square or more and 100ohm/square or less can be achieved. Since such a material provides ahigh light transmittance, the metal nanowire, the metal mesh, a carbonnanotube, graphene, or the like may be used for an electrode of thedisplay element, such as a pixel electrode or a common electrode.

Alternatively, one or more of layers each formed using a material havinga specific function, such as an anti-reflection layer, a light diffusionlayer, a microlens array, a prism sheet, a retardation plate, or apolarizing plate, (hereinafter referred to as “functional layers”) maybe provided on the outside of the substrate 111 or the substrate 121through which light 235 is emitted. As the anti-reflection layer, forexample, a circularly polarizing plate or the like can be used. With thefunctional layer, a display device having a higher display quality canbe achieved. Moreover, power consumption of the display device can bereduced. As the functional layer, a substrate including a touch sensormay be provided to overlap with the display device 100.

FIG. 20B is a cross-sectional view of the display device 100 having atop-emission structure including a functional layer 161. The functionallayer 161 is provided on an outer surface of the substrate 121. Notethat in the case where the display device 100 has a bottom-emissionstructure, the functional layer 161 may be provided on an outer surfaceof the substrate 111. In the case where the display device 100 has adual-emission structure, the functional layers 161 may be provided onthe outer surfaces of the substrate 111 and the substrate 121.

For the substrate 111 or the substrate 121, a material having a specificfunction may be used. For example, a circularly polarizing plate may beused as the substrate 111 or the substrate 121. Alternatively, forexample, the substrate 111 or the substrate 121 may be formed using aretardation plate, and a polarizing plate may be provided so as tooverlap with the substrate. As another example, the substrate 111 or thesubstrate 121 may be formed using a prism sheet, and a circularlypolarizing plate may be provided so as to overlap with the substrate.With the use of the material having a specific function for thesubstrate 111 or the substrate 121, improvement of display quality andreduction of the manufacturing cost can be achieved.

In the case where the display device performs monochrome display or thecase where the display device is used as a lighting device, the coloringlayer 266 is not necessarily provided as illustrated in FIG. 21.According to the case, the light-blocking layer 264 and the overcoatlayer 268 may be omitted. In the case where the light-emitting element125 has a micro optical resonator structure to be described later, thecoloring layer 266 may be omitted. A semiconductor chip 162 may beprovided over the external electrode 124.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 3

In this embodiment, a structure example of a transistor that can be usedin place of the transistor described in the above embodiments will bedescribed with reference to FIG. 22A1 to FIG. 27C.

Note that a conductive layer such as an electrode, a semiconductorlayer, an insulating layer, a substrate, and the like used in atransistor disclosed in this embodiment can be formed using a materialand a method disclosed in any of the other embodiments. For example, thesemiconductor layer 242 can be formed using a material and a methodsimilar to those of the semiconductor layer 208.

[Bottom-Gate Transistor]

A transistor 410 shown in FIG. 22A1 as an example is achannel-protective transistor that is a type of bottom-gate transistor.The transistor 410 includes an electrode 246 over a substrate 271 withan insulating layer 109 positioned therebetween. The transistor 410includes a semiconductor layer 242 over the electrode 246 with aninsulating layer 116 positioned therebetween. The electrode 246 canfunction as a gate electrode, and be formed using a material and amethod similar to those of the gate electrode 206. The insulating layer116 can function as a gate insulating layer, and be formed using amaterial and a method similar to those of the gate insulating layer 207.

The transistor 410 includes an insulating layer 209 that can function asa channel protective layer over a channel formation region in thesemiconductor layer 242. The insulating layer 209 can be formed using amaterial and a method similar to those of the insulating layer 116. Thetransistor 410 includes an electrode 244 and an electrode 245 which arepartly in contact with the semiconductor layer 242 and over theinsulating layer 116. Part of the electrode 244 and part of theelectrode 245 are formed over the insulating layer 209. One of theelectrode 244 and the electrode 245 functions as a source electrode, andthe other thereof functions as a drain electrode. The electrode 244 andthe electrode 245 can be formed using a material and a method similar tothose of the source electrode 209 a and the drain electrode 209 b.

With the insulating layer 209 provided over the channel formationregion, the semiconductor layer 242 can be prevented from being exposedat the time of forming the electrode 244 and the electrode 245. Thus,the semiconductor layer 242 can be prevented from being reduced inthickness at the time of forming the electrode 244 and the electrode245. With one embodiment of the present invention, a transistor withfavorable electrical characteristics can be provided.

The transistor 410 includes the layer 210 over the electrode 244, theelectrode 245, and the insulating layer 209.

In the case where an oxide semiconductor is used for the semiconductorlayer 242, a material which is capable of removing oxygen from part ofthe semiconductor layer 242 to generate oxygen vacancies is preferablyused for regions of the electrodes 224 and 225 which are in contact withat least the semiconductor layer 242. The carrier concentration of theregions of the semiconductor layer 242 in which oxygen vacancies aregenerated is increased, so that the regions become n-type regions (n⁺layers). Accordingly, the regions can function as a source region and adrain region. Examples of the material which is capable of removingoxygen from the oxide semiconductor to generate oxygen vacancies includetungsten and titanium.

Formation of the source region and the drain region in the semiconductorlayer 242 makes it possible to reduce contact resistance between thesemiconductor layer 242 and each of the electrodes 224 and 225.Accordingly, the electrical characteristics of the transistor, such asthe field-effect mobility and the threshold voltage, can be favorable.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 242, a layer that functions as an n-typesemiconductor or a p-type semiconductor is preferably provided betweenthe semiconductor layer 242 and the electrode 224 and between thesemiconductor layer 242 and the electrode 225. The layer that functionsas an n-type semiconductor or a p-type semiconductor can function as asource region or a drain region in a transistor.

A transistor 411 illustrated in FIG. 22A2 is different from thetransistor 410 in that an electrode 213 that can function as a back gateelectrode is provided over the insulating layer 210. The electrode 213can be formed using a material and a method that are similar to those ofthe gate electrode 206.

In general, the back gate electrode is formed using a conductive layerand positioned so that the channel formation region of the semiconductorlayer is provided between the gate electrode and the back gateelectrode. Thus, the back gate electrode can function in a mannersimilar to that of the gate electrode. The potential of the back gateelectrode may be the same as that of the gate electrode or may be a GNDpotential or a predetermined potential. By changing a potential of theback gate electrode independently of the potential of the gateelectrode, the threshold voltage of the transistor can be changed.

The electrodes 246 and 213 can both function as a gate electrode. Thus,the insulating layers 116, 209, and 210 can all function as a gateinsulating layer.

In the case where one of the electrode 246 and the electrode 213 issimply referred to as a “gate electrode”, the other can be referred toas a “back gate electrode”. For example, in the transistor 411, in thecase where the electrode 213 is referred to as a “gate electrode”, theelectrode 246 is referred to as a “back gate electrode”. In the casewhere the electrode 213 is used as a “gate electrode”, the transistor411 is a kind of bottom-gate transistor. Furthermore, one of theelectrode 246 and the electrode 213 may be referred to as a “first gateelectrode”, and the other may be referred to as a “second gateelectrode”.

By providing the electrode 246 and the electrode 213 with thesemiconductor layer 242 provided therebetween and setting the potentialsof the electrode 246 and the electrode 213 to be the same, a region ofthe semiconductor layer 242 through which carriers flow is enlarged inthe film thickness direction; thus, the number of transferred carriersis increased. As a result, the on-state current and the field-effectmobility of the transistor 411 are increased.

Therefore, the transistor 411 has large on-state current for the areaoccupied thereby. That is, the area occupied by the transistor 411 canbe small for required on-state current. With one embodiment of thepresent invention, the area occupied by a transistor can be reduced.Therefore, with one embodiment of the present invention, a highlyintegrated semiconductor device can be provided.

Furthermore, the gate electrode and the back gate electrode are formedusing conductive layers and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, anelectric field blocking function against static electricity and thelike). When the back gate electrode is formed larger than thesemiconductor layer such that the semiconductor layer is covered withthe back gate electrode, the electric field blocking function can beenhanced.

Since the electrode 246 and the electrode 213 each have a function ofblocking an electric field generated outside, charges of chargedparticles and the like generated on the insulating layer 109 side orabove the electrode 213 do not influence the channel formation region inthe semiconductor layer 242. Therefore, degradation in a stress test(e.g., a negative gate bias temperature (−GBT) stress test in whichnegative charges are applied to a gate) can be reduced, and changes inthe rising voltages of on-state current at different drain voltages canbe suppressed. Note that this effect is caused when the electrodes 246and 213 have the same potential or different potentials.

The BT stress test is one kind of accelerated test and can evaluate, ina short time, a change by long-term use (i.e., a change over time) incharacteristics of transistors. In particular, the change in thresholdvoltage of the transistor between before and after the BT stress test isan important indicator when examining the reliability of the transistor.If the change in the threshold voltage between before and after the BTstress test is small, the transistor has higher reliability.

By providing the electrode 246 and the electrode 213 and setting thepotentials of the electrode 246 and the electrode 213 to be the same,the change in threshold voltage is reduced. Accordingly, variation inelectrical characteristics among a plurality of transistors is alsoreduced.

The transistor including the back gate electrode has a smaller change inthreshold voltage between before and after a positive GBT stress test inwhich positive charges are applied to a gate than a transistor includingno back gate electrode.

When the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

A transistor 420 shown in FIG. 22B1 as an example is achannel-protective transistor that is a type of bottom-gate transistor.The transistor 420 has substantially the same structure as thetransistor 410 but is different from the transistor 410 in that theinsulating layer 209 covers the semiconductor layer 242. Thesemiconductor layer 242 is electrically connected to the electrode 244in an opening which is formed by selectively removing part of theinsulating layer 209 overlapping with the semiconductor layer 242. Thesemiconductor layer 242 is electrically connected to the electrode 245in the opening which is formed by selectively removing part of theinsulating layer 209 overlapping with the semiconductor layer 242. Aregion of the insulating layer 209 which overlaps with the channelformation region can function as a channel protective layer.

A transistor 421 illustrated in FIG. 22B2 is different from thetransistor 420 in that the electrode 213 that can function as a backgate electrode is provided over the insulating layer 210.

With the insulating layer 209, the semiconductor layer 242 can beprevented from being exposed at the time of forming the electrode 244and the electrode 245. Thus, the semiconductor layer 242 can beprevented from being reduced in thickness at the time of forming theelectrode 244 and the electrode 245.

The length between the electrode 244 and the electrode 246 and thelength between the electrode 245 and the electrode 246 in thetransistors 420 and 421 are longer than those in the transistors 410 and411. Thus, the parasitic capacitance generated between the electrode 244and the electrode 246 can be reduced. Moreover, the parasiticcapacitance generated between the electrode 245 and the electrode 246can be reduced.

A transistor 425 illustrated in FIG. 22C1 is a channel-etched transistorthat is a type of bottom-gate transistor. In the transistor 425, theelectrodes 244 and 245 are formed without using the insulating layer209, and therefore the semiconductor layer 242 might be partly exposedand etched when the electrodes 244 and 245 are formed. However, sincethe insulating layer 209 is not provided, the productivity of thetransistor can be increased.

A transistor 426 illustrated in FIG. 22C2 is different from thetransistor 420 in that an electrode 213 that can function as a back gateelectrode is provided over the insulating layer 210.

[Top-Gate Transistor]

A transistor 430 shown in FIG. 23A1 as an example is a type of top-gatetransistor. The transistor 430 includes the semiconductor layer 242 overthe substrate 271 with the insulating layer 109 positioned therebetween;the electrode 244 in contact with part of the semiconductor layer 242and the electrode 245 in contact with part of the semiconductor layer242, over the semiconductor layer 242 and the insulating layer 109; theinsulating layer 116 over the semiconductor layer 242 and the electrodes244 and 245; and the electrode 246 over the insulating layer 116.

Since, in the transistor 430, the electrode 246 overlaps with neitherthe electrode 244 nor the electrode 245, the parasitic capacitancegenerated between the electrode 246 and the electrode 244 and theparasitic capacitance generated between the electrode 246 and theelectrode 245 can be reduced. After the formation of the electrode 246,an impurity element 255 is added to the semiconductor layer 242 usingthe electrode 246 as a mask, so that an impurity region can be formed inthe semiconductor layer 242 in a self-aligned manner (see FIG. 23A3).With one embodiment of the present invention, a transistor withfavorable electrical characteristics can be provided.

The introduction of the impurity element 255 can be performed with anion implantation apparatus, an ion doping apparatus, or a plasmatreatment apparatus.

As the impurity element 255, for example, at least one element of aGroup 13 element and a Group 15 element can be used. In the case wherean oxide semiconductor is used for the semiconductor layer 242, it ispossible to use at least one kind of element of a rare gas, hydrogen,and nitrogen as the impurity element 255.

A transistor 431 illustrated in FIG. 23A2 is different from thetransistor 430 in that the electrode 213 and an insulating layer 217 areincluded. The transistor 431 includes the electrode 213 formed over theinsulating layer 109 and the insulating layer 217 formed over theelectrode 213. As described above, the electrode 213 can function as aback gate electrode. Thus, the insulating layer 217 can function as agate insulating layer. The insulating layer 217 can be formed using amaterial and a method that are similar to those of the insulating layer205.

The transistor 431 as well as the transistor 411 has large on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 431 can be small for required on-state current. With oneembodiment of the present invention, the area occupied by a transistorcan be reduced. Therefore, with one embodiment of the present invention,a semiconductor device having a high degree of integration can beprovided.

A transistor 440 shown in FIG. 23B1 as an example is a type of top-gatetransistor. The transistor 440 is different from the transistor 430 inthat the semiconductor layer 242 is formed after the formation of theelectrode 244 and the electrode 245. A transistor 441 shown in FIG. 23B2as an example is different from the transistor 440 in that it includesthe electrode 213 and the insulating layer 217. Thus, in the transistors440 and 441, part of the semiconductor layer 242 is formed over theelectrode 244 and another part of the semiconductor layer 242 is formedover the electrode 245.

The transistor 441 as well as the transistor 411 has large on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 441 can be small for required on-state current. With oneembodiment of the present invention, the area occupied by a transistorcan be reduced. Therefore, with one embodiment of the present invention,a semiconductor device having a high degree of integration can beprovided.

In the transistors 440 and 441, after the formation of the electrode246, the impurity element 255 is added to the semiconductor layer 242using the electrode 246 as a mask, so that an impurity region can beformed in the semiconductor layer 242 in a self-aligned manner.

A transistor 442 illustrated in FIG. 24A1 as an example is a type oftop-gate transistor. The transistor 442 includes the electrode 244 andthe electrode 245 over the insulating layer 210. The electrode 244 andthe electrode 245 are electrically connected to the semiconductor layer242 through the openings formed in the insulating layer 210.

In the transistor 442, the insulating layer 116 in a region that doesnot overlap with the electrode 246 is partly removed. The insulatinglayer 116 included in the transistor 442 is partly extended across theends of the electrode 246.

The impurity element 255 is added to the semiconductor layer 242 usingthe electrode 246 and the insulating layer 116 as masks, so that animpurity region can be formed in the semiconductor layer 242 in aself-aligned manner (see FIG. 24A3).

At this time, the impurity element 255 is not added to the semiconductorlayer 242 in a region overlapping with the electrode 246, and theimpurity element 255 is added to the semiconductor layer 242 in a regionthat does not overlap with the electrode 246. The semiconductor layer242 in region to which the impurity element 255 is added through theinsulating layer 116 has a lower impurity concentration than a region towhich the impurity element 255 is added without through the insulatinglayer 116. Thus, a lightly doped drain (LDD) region is formed in thesemiconductor layer 242 in a region adjacent to the electrode 246 whenseen from the above.

A transistor 443 illustrated in FIG. 24A2 is different from thetransistor 442 in that the transistor 443 includes an electrode 223 overthe substrate 271. The electrode 223 and the semiconductor layer 242overlap with each other with the insulating layer 109 positionedtherebetween. The electrode 223 can function as a back gate electrode.

As in a transistor 444 illustrated in FIG. 24B1 and a transistor 445illustrated in FIG. 24B2, the insulating layer 116 in a region that doesnot overlap with the electrode 246 may be completely removed.Alternatively, as in a transistor 446 illustrated in FIG. 24C1 and atransistor 447 illustrated in FIG. 24C2, the insulating layer 116 exceptfor the insulating layer 116 overlapping the opening portion may be leftwithout being removed.

In the transistors 444 to 447, after the formation of the electrode 246,the impurity element 255 is added to the semiconductor layer 242 usingthe electrode 246 as a mask, so that an impurity region can be formed inthe semiconductor layer 242 in a self-aligned manner.

With one embodiment of the present invention, a transistor withfavorable electrical characteristics can be provided. Furthermore, withone embodiment of the present invention, a highly integratedsemiconductor device can be provided.

[S-Channel Transistor]

FIGS. 25A to 25C illustrate an example of a structure of a transistorincluding an oxide semiconductor layer as the semiconductor layer 242.In a transistor 450 illustrated in FIGS. 25A to 25C, a semiconductorlayer 242 b is formed over a semiconductor layer 242 a, and asemiconductor layer 242 c covers a top surface and a side surface of thesemiconductor layer 242 b and a side surface of the semiconductor layer242 a. FIG. 25A is a top view of the transistor 450. FIG. 25B is across-sectional view (in the channel length direction) taken along thedashed-dotted line X1-X2 in FIG. 25A. FIG. 25C is a cross-sectional view(in the channel width direction) taken along the dashed-dotted lineY1-Y2 in FIG. 25A.

Each of the semiconductor layer 242 a, the semiconductor layer 242 b,and the semiconductor layer 242 c is formed using a material containingeither In or Ga or both of them. Typical examples are an In—Ga oxide (anoxide containing In and Ga), an In—Zn oxide (an oxide containing In andZn), and an In-M-Zn oxide (an oxide containing In, an element M, and Zn:the element M is one or more kinds of elements selected from Al, Ti, Ga,Y, Zr, La, Ce, Nd, and Hf and corresponds to a metal element whosestrength of bonding with oxygen is higher than that of In).

The semiconductor layer 242 a and the semiconductor layer 242 c arepreferably formed using a material containing one or more kinds of metalelements contained in the semiconductor layer 242 b. With use of such amaterial, interface states at interfaces between the semiconductor layer242 a and the semiconductor layer 242 b and between the semiconductorlayer 242 c and the semiconductor layer 242 b are less likely to begenerated. Accordingly, carriers are not likely to be scattered orcaptured at the interfaces, which results in an improvement infield-effect mobility of the transistor. Further, threshold-voltagevariation of the transistor can be reduced. Thus, a semiconductor devicehaving favorable electrical characteristics can be obtained.

Each of the thicknesses of the semiconductor layer 242 a and thesemiconductor layer 242 c is greater than or equal to 3 nm and less thanor equal to 100 nm, preferably greater than or equal to 3 nm and lessthan or equal to 50 nm. The thickness of the semiconductor layer 242 bis greater than or equal to 3 nm and less than or equal to 200 nm,preferably greater than or equal to 3 nm and less than or equal to 100nm, further preferably greater than or equal to 3 nm and less than orequal to 50 nm.

In the case where the semiconductor layer 242 b is an In-M-Zn oxide andthe semiconductor layer 242 a and the semiconductor layer 242 c are eachan In-M-Zn oxide, the semiconductor layer 242 a and the semiconductorlayer 242 c each have the atomic ratio where In:M:Zn=x₁:y₁:z₁, and thesemiconductor layer 242 b has an atomic ratio where In:M:Zn=x₂:y₂:z₂,for example. In that case, the compositions of the semiconductor layer242 a, the semiconductor layer 242 c, and the semiconductor layer 242 bare determined so that y₁/x₁ is large than y₂/x₂. It is preferable thatthe compositions of the semiconductor layer 242 a, the semiconductorlayer 242 c, and the semiconductor layer 242 b are determined so thaty₁/x₁ is 1.5 times or more as large as y₂/x₂. It is further preferablethat the compositions of the semiconductor layer 242 a, thesemiconductor layer 242 c, and the semiconductor layer 242 b aredetermined so that y₁/x₁ is twice or more as large as y₂/x₂. It is stillfurther preferable that the compositions of the semiconductor layer 242a, the semiconductor layer 242 c, and the semiconductor layer 242 b aredetermined so that y₁/x₁ is three times or more as large as y₂/x₂. Atthis time, y₁ is preferably greater than or equal to x₁ in thesemiconductor layer 242 b, in which case stable electricalcharacteristics of a transistor can be achieved. However, when y₁ isthree times or more as large as x₁, the field-effect mobility of thetransistor is reduced; accordingly, y₁ is preferably smaller than threetimes x₁. When the semiconductor layer 242 a and the semiconductor layer242 c have the above compositions, the semiconductor layer 242 a and thesemiconductor layer 242 c can each be a layer in which oxygen vacanciesare less likely to be generated than that in the semiconductor layer 242b.

In the case where the semiconductor layer 242 a and the semiconductorlayer 242 c are each an In-M-Zn oxide, the percentages of contained Inand an element M, not taking Zn and O into consideration, is preferablyas follows: the content percentage of In is lower than 50 atomic % andthe percentage of M is higher than or equal to 50 atomic %. The contentpercentages of In and M are further preferably as follows: the contentpercentage of In is lower than 25 atomic % and the content percentage ofM is higher than or equal to 75 atomic %. In the case of using anIn-M-Zn oxide for semiconductor layer 242 b, the content percentages ofIn and element M, not taking Zn and O into consideration, are preferablysuch that the percentage of In is higher than or equal to 25 atomic %and the percentage of M is lower than 75 atomic %. The contentpercentages In and element M are further preferably such that thepercentage of In is higher than or equal to 34 atomic % and thepercentage of M is lower than 66 atomic %.

For example, an In—Ga—Zn oxide which is formed using a target having anatomic ratio of In:Ga:Zn=1:3:2, 1:3:4, 1:3:6, 1:6:4, or 1:9:6 or anIn—Ga oxide which is formed using a target having an atomic ratio ofIn:Ga=1:9 can be used for each of the semiconductor layer 242 a and thesemiconductor layer 242 c containing In or Ga. Furthermore, an In—Ga—Znoxide which is formed using a target having an atomic ratio ofIn:Ga:Zn=3:1:2, 1:1:1, 5:5:6, or 4:2:4.1 can be used for thesemiconductor layer 242 b. Note that the atomic ratio of each of thesemiconductor layers 242 a, 242 b, and 242 c may vary within a range of±20% of any of the above-described atomic ratios as an error.

In order to give stable electrical characteristics to the transistorincluding the semiconductor layer 242 b, it is preferable thatimpurities and oxygen vacancies in the semiconductor layer 242 b bereduced to obtained a highly purified semiconductor layer; accordingly,the semiconductor layer 242 b can be regarded as an intrinsic orsubstantially intrinsic semiconductor layer. Furthermore, it ispreferable that at least the channel formation region of thesemiconductor layer 242 b be regarded as an intrinsic or substantiallyintrinsic semiconductor layer.

Note that the substantially intrinsic oxide semiconductor layer refersto an oxide semiconductor layer in which the carrier density is lowerthan 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, or lower than 1×10¹³/cm³.

[Energy Band Structure of Oxide Semiconductor]

The function and effect of the semiconductor layer 242 that is a stackedlayer including the semiconductor layer 242 a, the semiconductor layer242 b, and the semiconductor layer 242 c is described with an energyband structure diagram shown in FIG. 29. FIG. 29 is the energy bandstructure diagram showing a portion along dashed-dotted line D1-D2 inFIG. 25B. Thus, FIG. 29 shows the energy band structure of a channelformation region of the transistor 450.

In FIG. 29, Ec382, Ec383 a, Ec383 b, Ec383 c, and Ec386 are the energiesof bottoms of the conduction band in the insulating layer 109, thesemiconductor layer 242 a, the semiconductor layer 242 b, thesemiconductor layer 242 c, and the insulating layer 116, respectively.

Here, a difference in energy between the vacuum level and the bottom ofthe conduction band (the difference is also referred to as “electronaffinity”) corresponds to a value obtained by subtracting an energy gapfrom a difference in energy between the vacuum level and the top of thevalence band (the difference is also referred to as an ionizationpotential). Note that the energy gap can be measured using aspectroscopic ellipsometer (UT-300 manufactured by HORIBA JOBIN YVONS.A.S.). The energy difference between the vacuum level and the top ofthe valence band can be measured using an ultraviolet photoelectronspectroscopy (UPS) device (VersaProbe manufactured by ULVAC-PHI, Inc.).

Note that an In—Ga—Zn oxide which is formed using a target having anatomic ratio of In:Ga:Zn=1:3:2 has an energy gap of approximately 3.5 eVand an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxidewhich is formed using a target having an atomic ratio of In:Ga:Zn=1:3:4has an energy gap of approximately 3.4 eV and an electron affinity ofapproximately 4.5 eV. An In—Ga—Zn oxide which is formed using a targethaving an atomic ratio of In:Ga:Zn=1:3:6 has an energy gap ofapproximately 3.3 eV and an electron affinity of approximately 4.5 eV.An In—Ga—Zn oxide which is formed using a target having an atomic ratioof In:Ga:Zn=1:6:2 has an energy gap of approximately 3.9 eV and anelectron affinity of approximately 4.3 eV. An In—Ga—Zn oxide which isformed using a target having an atomic ratio of In:Ga:Zn=1:6:8 has anenergy gap of approximately 3.5 eV and an electron affinity ofapproximately 4.4 eV. An In—Ga—Zn oxide which is formed using a targethaving an atomic ratio of In:Ga:Zn=1:6:10 has an energy gap ofapproximately 3.5 eV and an electron affinity of approximately 4.5 eV.An In—Ga—Zn oxide which is formed using a target having an atomic ratioof In:Ga:Zn=1:1:1 has an energy gap of approximately 3.2 eV and anelectron affinity of approximately 4.7 eV. An In—Ga—Zn oxide which isformed using a target having an atomic ratio of In:Ga:Zn=3:1:2 has anenergy gap of approximately 2.8 eV and an electron affinity ofapproximately 5.0 eV.

Since the insulating layer 109 and the insulating layer 116 areinsulators, Ec382 and Ec386 are closer to the vacuum level (have asmaller electron affinity) than Ec383 a, Ec383 b, and Ec383 c.

Further, Ec383 a is closer to the vacuum level than Ec383 b.Specifically, Ec383 a is preferably located closer to the vacuum levelthan Ec383 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4eV or less.

Further, Ec383 c is closer to the vacuum level than Ec383 b.Specifically, Ec383 c is preferably located closer to the vacuum levelthan Ec383 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4eV or less.

In the vicinity of the interface between the semiconductor layer 242 aand the semiconductor layer 242 b and the vicinity of the interfacebetween the semiconductor layer 242 b and the semiconductor layer 242 c,mixed regions are formed; thus, the energy of the bottom of theconduction band continuously changes. In other words, no state or fewstates exist at these interfaces.

Accordingly, electrons transfer mainly through the semiconductor layer242 b in the stacked-layer structure having the above energy bandstructure. Therefore, even when an interface state exists at aninterface between the semiconductor layer 242 a and the insulating layer107 or an interface between the semiconductor layer 242 c and theinsulating layer 116, the interface state hardly influences the transferof the electrons. In addition, the interface state does not exist orhardly exists at an interface between the semiconductor layer 242 a andthe semiconductor layer 242 b and at an interface between thesemiconductor layer 242 c and the semiconductor layer 242 b; thus,transfer of electrons are not prohibited in the region. Consequently,the transistor 450 having the above stacked oxide semiconductors canhave high field-effect mobility.

Note that although trap states 390 due to impurities or defects might beformed in the vicinity of the interface between the semiconductor layer242 a and the insulating layer 109 and in the vicinity of the interfacebetween the semiconductor layer 242 c and the insulating layer 116 asshown in FIG. 29, the semiconductor layer 242 b can be separated fromthe trap states owing to the existence of the semiconductor layer 242 aand the semiconductor layer 242 c.

In particular, in the transistor 450 described in this embodiment, anupper surface and a side surface of the semiconductor layer 242 b are incontact with the semiconductor layer 242 c, and a bottom surface of thesemiconductor layer 242 b is in contact with the semiconductor layer 242a. In this manner, the semiconductor layer 242 b is surrounded by thesemiconductor layer 242 a and the semiconductor layer 242 c, whereby theinfluence of the trap state can be further reduced.

However, in the case where an energy difference between Ec383 a or Ec383c and Ec383 b is small, electrons in the semiconductor layer 242 b mightreach the trap states by passing over the energy gap. The electrons aretrapped by the trap states, which generates a negative fixed charge atthe interface with the insulating layer, causing the threshold voltageof the transistor to be shifted in the positive direction.

Therefore, each of the energy differences between Ec383 a and Ec383 band between Ec383 c and Ec383 b is preferably set to be greater than orequal to 0.1 eV, further preferably greater than or equal to 0.15 eV, inwhich case a change in the threshold voltage of the transistor can bereduced and the transistor can have favorable electricalcharacteristics.

Each of the band gaps of the semiconductor layer 242 a and thesemiconductor layer 242 c is preferably larger than that of thesemiconductor layer 242 b.

With one embodiment of the present invention, a transistor with a smallvariation in electrical characteristics can be provided. Accordingly, asemiconductor device with a small variation in electricalcharacteristics can be provided. With one embodiment of the presentinvention, a transistor with high reliability can be provided.Accordingly, a semiconductor device with high reliability can beprovided.

An oxide semiconductor has a band gap of 2 eV or more; therefore, atransistor including an oxide semiconductor in a semiconductor layer inwhich a channel is formed has an extremely small amount of off-statecurrent. Specifically, the off-state current per micrometer of channelwidth at room temperature can be less than 1×10⁻²⁰ A, preferably lessthan 1×10⁻²² A, further preferably less than 1×10⁻²⁴ A. That is, theon/off ratio of the transistor can be greater than or equal to 20 digitsand less than or equal to 150 digits.

With one embodiment of the present invention, a transistor with lowpower consumption can be provided. Accordingly, a semiconductor deviceor an imaging device with low power consumption can be provided.

A transistor using an oxide semiconductor in a semiconductor layer (alsoreferred to as OS transistor) has a significantly low off-state current.Therefore, for example, when an OS transistor is used as the transistor431, the capacitor 233 can be small. Alternatively, parasiticcapacitance of the transistor or the like can be used instead of thecapacitor 233 without providing the capacitor 233. Therefore, an areaoccupied by the pixels 130 can be reduced, which leads to highdefinition of the display region 131, whereby the display quality of thedisplay device 100 can be improved. Moreover, power consumption of thedisplay device 100 can be reduced. In addition, the display device 100with high reliability can be provided.

The transistor 450 illustrated in FIGS. 25A to 25C is described again. Asemiconductor layer 242 b is provided over a projecting portion of theinsulating layer 109, in which case the electrode 243 can cover a sidesurface of the semiconductor layer 242 b. That is, the transistor 450has a structure in which the semiconductor layer 242 b is electricallysurrounded by an electric field of the electrode 243. Such a structureof a transistor in which a semiconductor layer where a channel is formedis electrically surrounded by an electric field of a conductive film isreferred to as a surrounded channel (s-channel) structure. A transistorhaving an s-channel structure is referred to as an s-channel transistor.

In the s-channel transistor, a channel is formed in the whole (bulk) ofthe semiconductor layer 242 b in some cases. In the s-channeltransistor, the drain current of the transistor can be increased, sothat a larger amount of on-state current can be obtained. Furthermore,the entire channel formation region of the semiconductor layer 242 b canbe depleted by the electric field of the electrode 243. Accordingly, theoff-state current of the s-channel transistor can be further reduced.

When the projecting portion of the insulating layer 109 is increased inheight, and the channel width is shortened, the effects of the s-channelstructure to increase the on-state current and reduce the off-statecurrent can be enhanced. Part of the semiconductor layer 242 a exposedin the formation of the semiconductor layer 242 b may be removed. Inthis case, the side surfaces of the semiconductor layer 242 a and thesemiconductor layer 242 b are aligned to each other in some cases.

As in a transistor 451 illustrated in FIGS. 26A to 26C, the electrode213 may be provided under the semiconductor layer 242 with an insulatinglayer positioned therebetween. FIG. 26A is a top view of a transistor451. FIG. 26B is a cross-sectional view taken along the dashed-dottedline X1-X2 in FIG. 26A. FIG. 26C is a cross-sectional view taken alongthe dashed-dotted line Y1-Y2 in FIG. 26A.

As in the transistor 452 illustrated in FIGS. 27A to 27C, a layer 214may be provided over the electrode 243. FIG. 27A is a top view of thetransistor 452. FIG. 27B is a cross-sectional view taken along thedashed-dotted line X1-X2 in FIG. 27A. FIG. 27C is a cross-sectional viewtaken along the dashed-dotted line Y1-Y2 in FIG. 27A.

Although the layer 214 is provided over the insulating layer 109 inFIGS. 27A to 27C, the layer 214 may be provided over the insulatinglayer 210. The layer 214 formed with a material with a light-blockingproperty can prevent a change in transistor characteristics, a decreasein reliability, or the like caused by light irradiation. In the casewhere the layer 214 is formed larger than at least the semiconductorlayer 242 b and covers the semiconductor layer 242 b, theabove-described effects can be enhanced. The layer 214 can be formedwith an organic material, an inorganic material, or a metal material. Inthe case where the layer 214 is formed with a conductive material, thelayer 214 may be supplied with voltage or may be electrically floating.

FIGS. 28A to 28C illustrate an example of a transistor with an s-channelstructure. A transistor 448 in FIGS. 28A to 28C has almost the samestructure as the transistor 447. In the transistor 448, thesemiconductor layer 242 is formed over the projecting portion of theinsulating layer 109. The transistor 448 is a kind of top-gatetransistor having a back-gate electrode. FIG. 28A is a top view of thetransistor 448. FIG. 28B is a cross-sectional view taken along thedashed-dotted line X1-X2 in FIG. 28A. FIG. 28C is a cross-sectional viewtaken along the dashed-dotted line Y1-Y2 in FIG. 28A.

FIGS. 28A to 28C illustrate an example in which an inorganicsemiconductor layer such as a silicon layer is used as the semiconductorlayer 242 in the transistor 448. In FIGS. 28A to 28C, the semiconductorlayer 242 includes a semiconductor layer 242 i overlapping with the gateelectrode 246, two semiconductor layers 242 t, and two semiconductorlayers 242 u. The semiconductor layer 242 i is sandwiched between thetwo semiconductor layers 242 t. The semiconductor layer 242 i and thetwo semiconductor layers 242 t are sandwiched between the twosemiconductor layers 242 u.

A channel is formed in the semiconductor layer 242 i when the transistor448 is on. Therefore, the semiconductor layer 242 i serves as a channelformation region. The semiconductor layers 242 t serve as lowconcentration impurity regions (i.e., LDD). The semiconductor layers 242u serve as high concentration impurity regions. Note that one or both ofthe two semiconductor layers 242 t are not necessarily provided. One ofthe two semiconductor layers 242 u serves as a source region, and theother semiconductor layer 242 u serves as a drain region.

The electrode 244 formed over the insulating layer 210 is electricallyconnected to one of the semiconductor layers 242 u through an opening247 c formed in the insulating layer 116 and the insulating layer 210.An electrode 245 formed over the insulating layer 210 is electricallyconnected to the other of the semiconductor layers 242 u through anopening 247 d formed in the insulating layer 116 and the insulatinglayer 210.

The electrode 243 formed over the insulating layer 116 is electricallyconnected to the electrode 223 through an opening 247 a and an opening247 b formed in the insulating layer 116 and the insulating layer 109.Therefore, the same potential is applied to the electrode 246 and theelectrode 223. One or both of the openings 247 a and 247 b are notnecessarily provided. In the case neither the opening 247 a nor theopening 247 b are provided, different potentials can be applied to theelectrode 223 and the electrode 246.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 4

In this embodiment, structure examples of a light-emitting element thatcan be applied to the light-emitting element 125 are described. Notethat an EL layer 320 described in this embodiment corresponds to the ELlayer 117 described in the above embodiment.

<Structure of Light-Emitting Element>

In a light-emitting element 330 illustrated in FIG. 30A, the EL layer320 is interposed between a pair of electrodes (an electrode 318 and anelectrode 322). Note that the electrode 318 is used as an anode and theelectrode 322 is used as a cathode as an example in the followingdescription of this embodiment.

The EL layer 320 includes at least a light-emitting layer and may have astacked-layer structure including a functional layer other than thelight-emitting layer. As the functional layer other than thelight-emitting layer, a layer containing a substance having a highhole-injection property, a substance having a high hole-transportproperty, a substance having a high electron-transport property, asubstance having a high electron-injection property, a bipolar substance(a substance having high electron- and hole-transport properties), orthe like can be used. Specifically, functional layers such as ahole-injection layer, a hole-transport layer, an electron-transportlayer, and an electron-injection layer can be used in combination asappropriate.

The light-emitting element 330 illustrated in FIG. 30A emits light whencurrent flows because of a potential difference applied between theelectrode 318 and the electrode 322 and holes and electrons arerecombined in the EL layer 320. That is, the light-emitting region isformed in the EL layer 320.

In the present invention, light emitted from the light-emitting element330 is extracted to the outside from the electrode 318 side or theelectrode 322 side. Therefore, one of the electrode 318 and theelectrode 322 is formed of a light-transmitting substance.

Note that a plurality of EL layers 320 may be stacked between theelectrode 318 and the electrode 322 as in a light-emitting element 331illustrated in FIG. 30B. In the case where n (n is a natural number of 2or more) layers are stacked, a charge generation layer 320 a ispreferably provided between an m-th EL layer 320 and an (m+1)-th ELlayer 320. Note that m is a natural number greater than or equal to 1and less than n.

The charge generation layer 320 a can be formed using a compositematerial of an organic compound and a metal oxide, a metal oxide, acomposite material of an organic compound and an alkali metal, analkaline earth metal, or a compound thereof; alternatively, thesematerials can be combined as appropriate. Examples of the compositematerial of an organic compound and a metal oxide include compositematerials of an organic compound and a metal oxide such as vanadiumoxide, molybdenum oxide, and tungsten oxide. As the organic compound, avariety of compounds can be used; for example, low molecular compoundssuch as an aromatic amine compound, a carbazole derivative, and aromatichydrocarbon and oligomers, dendrimers, and polymers of these lowmolecular compounds. As the organic compound, it is preferable to usethe organic compound which has a hole-transport property and has a holemobility of 10⁻⁶ cm²/Vs or higher. However, substances other than thesubstances given above may also be used as long as the substances havehole-transport properties higher than electron-transport properties.These materials used for the charge generation layer 320 a haveexcellent carrier-injection properties and carrier-transport properties;thus, the light-emitting element 330 can be driven with low current andwith low voltage.

Note that the charge generation layer 320 a may be formed with acombination of a composite material of an organic compound and a metaloxide with another material. For example, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a layer containing a compound of a substance selected fromsubstances with an electron-donating property and a compound with a highelectron-transport property. Moreover, a layer containing a compositematerial of the organic compound and the metal oxide may be combinedwith a transparent conductive film.

The light-emitting element 331 having such a structure is unlikely tosuffer the problem of energy transfer, quenching, or the like and has anexpanded choice of materials, and thus can easily have both highemission efficiency and a long lifetime. Moreover, it is easy to obtainphosphorescence from one light-emitting layer and fluorescence from theother light-emitting layer.

The charge generation layer 320 a has a function of injecting holes toone of the EL layers 320 that is in contact with the charge generationlayer 320 a and a function of injecting electrons to the other EL layer320 that is in contact with the charge generation layer 320 a, whenvoltage is applied between the electrode 318 and the electrode 322.

The light-emitting element 331 illustrated in FIG. 30B can provide avariety of emission colors by changing the type of the light-emittingsubstance used for the EL layer 320. In addition, a plurality oflight-emitting substances emitting light of different colors may be usedas the light-emitting substances, whereby light emission having a broadspectrum or white light emission can be obtained.

In the case of obtaining white light emission using the light-emittingelement 331 illustrated in FIG. 30B, as for the combination of aplurality of EL layers, a structure for emitting white light includingred light, green light, and blue light may be used; for example, thestructure may include a light-emitting layer containing a bluefluorescent substance as a light-emitting substance and a light-emittinglayer containing red and green phosphorescent substances aslight-emitting substances. Alternatively, a structure including alight-emitting layer emitting red light, a light-emitting layer emittinggreen light, and a light-emitting layer emitting blue light may beemployed. Further alternatively, with a structure includinglight-emitting layers emitting light of complementary colors, whitelight emission can be obtained. In a stacked-layer element including twolight-emitting layers in which light emitted from one of thelight-emitting layers and light emitted from the other light-emittinglayer have complementary colors to each other, the combinations ofcolors are as follows: blue and yellow, blue-green and red, and thelike.

Note that in the structure of the above-described stacked-layer element,by providing the charge generation layer between the stackedlight-emitting layers, the element can have a long lifetime in ahigh-luminance region while keeping the current density low. Inaddition, the voltage drop due to the resistance of the electrodematerial can be reduced, whereby uniform light emission in a large areais possible.

With a micro optical resonator (also referred to as microcavity)structure which allows light emitted from the EL layer 117 to resonate,lights with different wavelengths and narrowed spectra can be extractedeven when one EL layer 117 is used for different light-emitting elements125.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 5

In this embodiment, examples of an electronic device including thedisplay device of one embodiment of the present invention are describedwith reference to drawings.

Specific examples of the electronic device that uses the display deviceof one embodiment of the present invention are as follows: displaydevices of televisions, monitors, and the like, lighting devices,desktop and laptop personal computers, word processors, imagereproduction devices which reproduce still images and moving imagesstored in recording media such as digital versatile discs (DVDs),portable CD players, radios, tape recorders, headphone stereos, stereos,table clocks, wall clocks, cordless phone handsets, transceivers, mobilephones, car phones, portable game machines, tablet terminals, large gamemachines such as pachinko machines, calculators, portable informationterminals, electronic notebooks, e-book readers, electronic translators,audio input devices, video cameras, digital still cameras, electricshavers, high-frequency heating appliances such as microwave ovens,electric rice cookers, electric washing machines, electric vacuumcleaners, water heaters, electric fans, hair dryers, air-conditioningsystems such as air conditioners, humidifiers, and dehumidifiers,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, flashlights, electrical tools such as achain saw, smoke detectors, and medical equipment such as dialyzers.Other examples are as follows: industrial equipment such as guidelights, traffic lights, conveyor belts, elevators, escalators,industrial robots, power storage systems, and power storage devices forleveling the amount of power supply and smart grid. In addition, movingobjects and the like driven by electric motors using power from a powerstorage unit are also included in the category of electronic devices.Examples of the moving objects include electric vehicles (EV), hybridelectric vehicles (HEV) which include both an internal-combustion engineand a motor, plug-in hybrid electric vehicles (PHEV), tracked vehiclesin which caterpillar tracks are substituted for wheels of thesevehicles, motorized bicycles including motor-assisted bicycles,motorcycles, electric wheelchairs, golf carts, boats, ships, submarines,helicopters, aircrafts, rockets, artificial satellites, space probes,planetary probes, and spacecrafts.

In particular, as examples of electronic devices including the displaydevice of one embodiment of the present invention, the following can begiven: television devices (also referred to as televisions or televisionreceivers), monitors of computers or the like, digital cameras, digitalvideo cameras, digital photo frames, mobile phones (also referred to ascellular phones or mobile phone devices), portable game machines,portable information terminals, audio reproducing devices, large gamemachines such as pachinko machines, and the like.

In addition, a lighting device or a display device can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

FIG. 31A is an example of a mobile phone (e.g., a smartphone). A mobilephone 7400 includes a display portion 7402 that is incorporated in ahousing 7401. The mobile phone 7400 further includes operation buttons7403, an external connection port 7404, a speaker 7405, a microphone7406, and the like. The mobile phone 7400 is manufactured using thedisplay device of one embodiment of the present invention for thedisplay portion 7402.

The mobile phone 7400 illustrated in FIG. 31A includes a touch sensor inthe display portion 7402. When the display portion 7402 is touched witha finger or the like, data can be input into the mobile phone 7400.Furthermore, operations such as making a call and inputting a letter canbe performed by touch on the display portion 7402 with a finger or thelike.

With the operation buttons 7403, power ON/OFF can be switched. Inaddition, types of images displayed on the display portion 7402 can beswitched; for example, switching images from a mail creation screen to amain menu screen.

Here, the display portion 7402 includes the display device of oneembodiment of the present invention. Thus, the mobile phone can have acurved display portion and high reliability.

FIG. 31B illustrates an example of a mobile phone (e.g., smartphone). Amobile phone 7410 includes a housing 7411 provided with a displayportion 7412, a microphone 7416, a speaker 7415, a camera 7417, anexternal connection portion 7414, an operation button 7413, and thelike. In the case where a display device of one embodiment of thepresent invention is manufactured using a flexible substrate, thedisplay device can be used for the display portion 7412 with a curvedsurface.

When the display portion 7412 of the cellular phone 7410 illustrated inFIG. 31B is touched with a finger or the like, data can be input to thecellular phone 7410. Operations such as making a call and creating ane-mail can be performed by touching the display portion 7412 with afinger or the like.

There are mainly three screen modes of the display portion 7412. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7412 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7412.

The screen modes can be switched depending on the kind of imagesdisplayed on the display portion 7412. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode may be switched to the display mode. When the signal isa signal of text data, the screen mode may be switched to the inputmode.

In the input mode, if a touch sensor in the display portion 7412 judgesthat the input by touch on the display portion 7412 is not performed fora certain period, the screen mode may be switched from the input mode tothe display mode.

When a detection device including a sensor (e.g., a gyroscope or anacceleration sensor) is provided inside the mobile phone 7410, thedirection of display on the screen of the display portion 7412 can beautomatically changed by determining the orientation of the mobile phone7410 (whether the mobile phone is placed horizontally or vertically).Furthermore, the direction of display on the screen can be changed bytouch on the display portion 7412 or operation with the operation button7413 of the housing 7411.

FIG. 31C is an example of a wristband-type display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102,operation buttons 7103, and a transceiver 7104.

The portable display device 7100 can receive a video signal with thetransceiver 7104 and can display the received video on the displayportion 7102. In addition, with the transceiver 7104, the portabledisplay device 7100 can send an audio signal to another receivingdevice.

With the operation button 7103, power ON/OFF, switching displayedvideos, adjusting volume, and the like can be performed.

Here, the display portion 7102 includes the display device of oneembodiment of the present invention. Thus, the portable display devicecan have a curved display portion and high reliability.

FIGS. 31D to 31F show examples of lighting devices. Lighting devices7200, 7210, and 7220 each include a stage 7201 provided with anoperation switch 7203 and a light-emitting portion supported by thestage 7201.

The lighting device 7200 illustrated in FIG. 31D includes alight-emitting portion 7202 with a wave-shaped light-emitting surfaceand thus is a good-design lighting device.

A light-emitting portion 7212 included in the lighting device 7210illustrated in FIG. 31E has two convex-curved light-emitting portionssymmetrically placed. Thus, light radiates from the lighting device 7210in all directions.

The lighting device 7220 illustrated in FIG. 31F includes aconcave-curved light-emitting portion 7222. This is suitable forilluminating a specific range because light emitted from thelight-emitting portion 7222 is collected to the front of the lightingdevice 7220.

The light-emitting portion included in each of the lighting devices7200, 7210, and 7220 is flexible; thus, the light-emitting portion canbe fixed on a plastic member, a movable frame, or the like so that anemission surface of the light-emitting portion can be curved freelydepending on the intended use.

The light-emitting portions included in the lighting devices 7200, 7210,and 7220 each include the display device of one embodiment of thepresent invention. Thus, the light-emitting portions can be curved orbent into any shape and the lighting devices can have high reliability.

FIG. 32A shows an example of a portable display device. A display device7300 includes a housing 7301, a display portion 7302, operation buttons7303, a display portion pull 7304, and a control portion 7305.

The display device 7300 includes the rolled flexible display portion7302 in the cylindrical housing 7301.

The display device 7300 can receive a video signal with the controlportion 7305 and can display the received video on the display portion7302. In addition, a power storage device is included in the controlportion 7305. Moreover, a connector may be included in the controlportion 7305 so that a video signal or power can be supplied directly.

With the operation buttons 7303, power ON/OFF, switching of displayedvideos, and the like can be performed.

FIG. 32B illustrates a state where the display portion 7302 is pulledout with the display portion pull 7304. Videos can be displayed on thedisplay portion 7302 in this state. Furthermore, the operation buttons7303 on the surface of the housing 7301 allow one-handed operation.

Note that a reinforcement frame may be provided for an edge of thedisplay portion 7302 in order to prevent the display portion 7302 frombeing curved when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

The display portion 7302 includes the display device of one embodimentof the present invention. Thus, the display portion 7302 is a displaydevice which is flexible and highly reliable, which makes the displaydevice 7300 lightweight and highly reliable.

FIGS. 33A and 33B show a double foldable tablet terminal 9600 as anexample. FIG. 33A illustrates the tablet terminal 9600 which isunfolded. The tablet terminal 9600 includes a housing 9630, a displayportion 9631, a display mode switch 9626, a power switch 9627, apower-saving mode switch 9625, a clasp 9629, and an operation switch9628.

The housing 9630 includes a housing 9630 a and a housing 9630 b, whichare connected with a hinge portion 9639. The hinge portion 9639 makesthe housing 9630 double foldable.

The display portion 9631 is provided on the housing 9630 a, the housing9630 b, and the hinge portion 9639. By the use of the display devicedisclosed in this specification and the like for the display portion9631, the tablet terminal in which the display portion 9631 is foldableand which has high reliability can be provided.

Part of the display portion 9631 can be a touchscreen region 9632 anddata can be input when a displayed operation key 9638 is touched. Astructure can be employed in which half of the display portion 9631 hasonly a display function and the other half has a touchscreen function.The whole display portion 9631 may have a touchscreen function. Forexample, keyboard buttons may be displayed on the entire region of thedisplay portion 9631 so that the display portion 9631 can be used as adata input terminal.

The display mode switch 9626 can switch the display between a portraitmode and a landscape mode, and between monochrome display and colordisplay, for example. The power-saving mode switch 9625 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. Another detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal, in addition to the opticalsensor.

FIG. 33B illustrates the tablet terminal 9600 which is folded. Thetablet terminal 9600 includes the housing 9630, a solar cell 9633, and acharge and discharge control circuit 9634. As an example, FIG. 33Billustrates the charge and discharge control circuit 9634 including abattery 9635 and a DC-DC converter 9636.

By including the display device of one embodiment of the presentinvention, the display portion 9631 is foldable. Since the tabletterminal 9600 is double foldable, the housing 9630 can be closed whenthe tablet terminal is not in use, for example; thus, the tabletterminal is highly portable. Moreover, since the display portion 9631can be protected when the housing 9630 is closed, the tablet terminalcan have high durability and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 33A and 33B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 ispreferably provided on one or both surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently. When a lithiumion battery is used as the battery 9635, there is an advantage ofdownsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 33B is described with reference to a blockdiagram of FIG. 33C. FIG. 33C illustrates the solar cell 9633, thebattery 9635, the DC-DC converter 9636, a converter 9637, switches SW1to SW3, and the display portion 9631. The battery 9635, the DC-DCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 33B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DC-DC converter 9636 so as to be voltage for charging thebattery 9635. Then, when power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration unit, the power generation unit is not particularly limited,and the battery 9635 may be charged by another power generation unitsuch as a piezoelectric element or a thermoelectric conversion element(Peltier element). For example, the battery 9635 may be charged using anon-contact power transmission module that transmits and receives powerwirelessly (without contact) or using another charge unit incombination.

It is needless to say that one embodiment of the present invention isnot limited to the above-described electronic devices and lightingdevices as long as the display device of one embodiment of the presentinvention is included.

FIGS. 34A to 34C illustrate a foldable portable information terminal9310 as an example of an electronic device. FIG. 34A illustrates theportable information terminal 9310 that is opened. FIG. 34B illustratesthe portable information terminal 9310 that is being opened or beingfolded. FIG. 34C illustrates the portable information terminal 9310 thatis folded. The portable information terminal 9310 includes a displaypanel 9316, housings 9315, and hinges 9313. The portable informationterminal 9310 is highly portable when folded. When the portableinformation terminal 9310 is opened, a seamless large display region isobtained; thus, the display image is highly browsable.

The display panel 9316 included in the portable information terminal9310 is supported by the three housings 9315 joined together by thehinges 9313. The display panel 9316 can be folded at the hinges 9313.The portable information terminal 9310 can be reversibly changed inshape from an opened state to a folded state. The display device of oneembodiment of the present invention can be used for the display panel9316. For example, a display device that can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm can be used. The display panel 9316 may include a touch sensor.

Note that in one embodiment of the present invention, a sensor thatsenses whether the display panel 9316 is in a folded state or anunfolded state may be used. The operation of a folded portion (or aportion that becomes invisible by a user by folding) of the displaypanel 9316 may be stopped by a control device through the acquisition ofdata indicating the folded state of the touch panel. Specifically,display of the portion may be stopped. In the case where a touch sensoris included, detection by the touch sensor may be stopped.

Similarly, the control device of the display panel 9316 may acquire dataindicating the unfolded state of the display panel 9316 to resumedisplaying and sensing by the touch sensor.

FIGS. 34D and 34E each illustrate a foldable portable informationterminal 9320. FIG. 34D illustrates the portable information terminal9320 that is folded so that a display portion 9322 is on the outside.FIG. 34E illustrates the portable information terminal 9320 that isfolded so that the display portion 9322 is on the inside. When theportable information terminal 9320 is not used, the portable informationterminal 9320 is folded so that a non-display portion 9325 faces theoutside, whereby the display portion 9322 can be prevented from beingcontaminated or damaged. The display device of one embodiment of thepresent invention can be used for the display portion 9322.

FIG. 34F is a perspective view illustrating an external shape of aportable information terminal 9330. FIG. 34G is a top view of theportable information terminal 9330. FIG. 34H is a perspective viewillustrating an external shape of a portable information terminal 9340.

The portable information terminals 9330 and 9340 each function as, forexample, one or more of a telephone set, a notebook, and an informationbrowsing system. Specifically, the portable information terminals 9330and 9340 each can be used as a smartphone.

The portable information terminals 9330 and 9340 can display charactersand image information on their plurality of surfaces. For example, oneor more operation buttons 9339 can be displayed on the front surface(FIG. 34F). In addition, information 9337 indicated by dashed rectanglescan be displayed on the top surface (FIG. 34G) or on the side surface(FIG. 34H). Examples of the information 9337 include notification from asocial networking service (SNS), display indicating reception of ane-mail or an incoming call, the title of an e-mail or the like, thesender of an e-mail or the like, the date, the time, remaining battery,and the reception strength of an antenna. Alternatively, the operationbuttons 9339, an icon, or the like may be displayed in place of theinformation 9337. Although FIGS. 34F and 34G illustrate an example inwhich the information 9337 is displayed at the top and side surfaces,one embodiment of the present invention is not limited thereto. Theinformation 9337 may be displayed, for example, on the bottom or rearsurface.

For example, a user of the portable information terminal 9330 can seethe display (here, the information 9337) with the portable informationterminal 9330 put in a breast pocket of his/her clothes.

Specifically, a caller's phone number, name, or the like of an incomingcall is displayed on the front surface of the portable informationterminal 9330. Thus, the user can see the display without taking out theportable information terminal 9330 from the pocket and decide whether toanswer the call.

The display device of one embodiment of the present invention can beused for a display portion 9333 mounted in each of a housing 9335 of theportable information terminal 9330 and a housing 9336 of the portableinformation terminal 9340. One embodiment of the present invention canprovide a highly reliable display device having a curved display portionwith a high yield.

As in a portable information terminal 9345 illustrated in FIG. 34I, datamay be displayed on three or more surfaces. Here, data 9355, data 9356,and data 9357 are displayed on different surfaces.

The display device of one embodiment of the present invention can beused for a display portion 9358 included in a housing 9354 of theportable information terminal 9345. One embodiment of the presentinvention can provide a highly reliable display device having a curveddisplay portion with a high yield.

FIG. 35A is an external view of an automobile 9700. FIG. 35B illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The display device of one embodiment of the present invention can beused in a display portion or the like of the automobile 9700. Forexample, the display device of one embodiment of the present inventioncan be used in display portions 9710 to 9715 illustrated in FIG. 35B.

The display portion 9710 and the display portion 9711 are displaydevices provided in an automobile windshield. The display device of oneembodiment of the present invention can be a see-through display device,through which the opposite side can be seen, by using alight-transmitting conductive material for its electrodes. Such asee-through display device does not hinder driver's vision duringdriving the automobile 9700. Therefore, the display device of oneembodiment of the present invention can be provided in the windshield ofthe automobile 9700. Note that in the case where a transistor or thelike for driving the display device is provided in the display device, atransistor having light-transmitting properties, such as an organictransistor using an organic semiconductor material or a transistor usingan oxide semiconductor, is preferably used.

The display portion 9712 is a display device provided on a pillarportion. For example, an image taken by an imaging unit provided in thecar body is displayed on the display portion 9712, whereby the viewhindered by the pillar portion can be compensated. The display portion9713 is a display device provided on the dashboard. For example, animage taken by an imaging unit provided in the car body is displayed onthe display portion 9713, whereby the view hindered by the dashboard canbe compensated. That is, by displaying an image taken by an imaging unitprovided on the outside of the automobile, blind areas can be eliminatedand safety can be increased. Displaying an image to compensate for thearea which a driver cannot see, makes it possible for the driver toconfirm safety easily and comfortably.

FIG. 36 illustrates the inside of a car in which bench seats are usedfor a driver seat and a front passenger seat. A display portion 9721 isa display device provided in a door portion. For example, an image takenby an imaging unit provided in the car body is displayed on the displayportion 9721, whereby the view hindered by the door can be compensated.A display portion 9722 is a display device provided in a steering wheel.A display portion 9723 is a display device provided in the middle of aseating face of the bench seat. Note that the display device can be usedas a seat heater by providing the display device on the seating face orbackrest and by using heat generation of the display device as a heatsource.

The display portion 9714, the display portion 9715, and the displayportion 9722 can provide a variety of kinds of information such asnavigation data, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be changed freely bya user as appropriate. The information listed above can also bedisplayed on the display portions 9710 to 9713, 9721, and 9723. Thedisplay portions 9710 to 9715 and 9721 to 9723 can also be used aslighting devices. The display portions 9710 to 9715 and 9721 to 9723 canalso be used as heating devices. This embodiment can be implemented inan appropriate combination with any of the structures described in theother embodiments.

REFERENCE NUMERALS

100: display device, 101: substrate, 107: insulating layer, 109:insulating layer, 111: substrate, 112: bonding layer, 113: separationlayer, 114: partition wall, 115: electrode, 116: insulating layer, 117:EL layer, 118: electrode, 119: insulating layer, 120: bonding layer,121: substrate, 122: opening, 123: anisotropic conductive connectionlayer, 124: external electrode, 125: light-emitting element, 128:opening, 129: opening, 130: pixel, 131: display region, 132: circuit,133: circuit, 134: transistor, 135: scan line, 136: signal line, 137:pixel circuit, 140: pixel, 141: element formation substrate, 142:bonding layer, 143: separation layer, 145: insulating layer, 147: layer,152: circuit, 153: circuit, 161: functional layer, 162: semiconductorchip, 170: region, 171: element substrate, 181: counter substrate, 191:metallic mold, 192: depressed portion, 193: metallic mold, 194:depressed portion, 195: filler, 200: display device, 205: insulatinglayer, 206: gate electrode, 207: gate insulating layer, 208:semiconductor layer, 209: insulating layer, 210: insulating layer, 211:insulating layer, 213: electrode, 214: layer, 216: terminal electrode,217: insulating layer, 219: wiring, 231: display region, 232:transistor, 233: capacitor, 235: light, 242: semiconductor layer, 243:electrode, 244: electrode, 245: electrode, 246: electrode, 252:transistor, 255: impurity element, 264: light-blocking layer, 266:coloring layer, 268: overcoat layer, 270: touch sensor, 271: substrate,272: electrode, 273: insulating layer, 274: electrode, 275: insulatinglayer, 318: electrode, 320: EL layer, 322: electrode, 330:light-emitting element, 331: light-emitting element, 382: Ec, 386: Ec,390: trap state, 410: transistor, 411: transistor, 420: transistor, 421:transistor, 430: transistor, 431: transistor, 432: liquid crystalelement, 434: transistor, 435: node, 436: node, 437: node, 440:transistor, 441: transistor, 450: transistor, 451: transistor, 452:transistor, 7100: portable display device, 7101: housing, 7102: displayportion, 7103: operation button, 7104: sending and receiving device,7200: lighting device, 7201: stage, 7202: light-emitting portion, 7203:operation switch, 7210: lighting device, 7212: light-emitting portion,7220: lighting device, 7222: light-emitting portion, 7300: displaydevice, 7301: housing, 7302: display portion, 7303: operation button,7304: display portion pull, 7305: control portion, 7400: mobile phonedevice, 7401: housing, 7402: display portion, 7403: operation button,7404: external connection port, 7405: speaker, 7406: microphone, 7410:mobile phone device, 7411: housing, 7412: display portion, 7413:operation button, 7414: external connection portion, 7415: speaker,7416: microphone, 7417: camera, 9310: portable information terminal,9313: hinge, 9315: housing, 9316: display panel, 9320: portableinformation terminal, 9322: display portion, 9325: non-display portion,9330: portable information terminal, 9333: display portion, 9335:housing, 9336: housing, 9337: information, 9339: operation button, 9340:portable information terminal, 9345: portable information terminal,9354: housing, 9355: information, 9356: information, 9357: information,9358: display portion, 9600: tablet terminal, 9625: switch, 9626:switch, 9627: power supply switch, 9628: operation switch, 9629: clasp,9630: housing, 9631: display portion, 9632: region, 9633: solar cell,9634: charge and discharge control circuit, 9635: battery, 9636: DC-DCconverter, 9637: converter, 9638: operation key, 9639: hinge portion,9700: automobile, 9701: car body, 9702: wheel, 9703: dashboard, 9704:light, 9710: display portion, 9711: display portion, 9712: displayportion, 9713: display portion, 9714: display portion, 9715: displayportion, 9721: display portion, 9722: display portion, 9723: displayportion, 130B: pixel, 130G: pixel, 130R: pixel, 130Y: pixel, 209 a:source electrode, 209 b: drain electrode, 242 a: semiconductor layer,242 b: semiconductor layer, 242 c: semiconductor layer, 320 a:charge-generation layer, 383 a: Ec, 383 b: Ec, 383 c: Ec, 9630 a:housing, 9630 b: housing

This application is based on Japanese Patent Application serial no.2014-152036 filed with Japan Patent Office on Jul. 25, 2014, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: a firstflexible substrate; a first adhesive layer over the first flexiblesubstrate; a first insulating layer over the first adhesive layer; adisplay portion having a light-emitting element over the firstinsulating layer; a driver circuit over the first insulating layer; asecond adhesive layer over the display portion and the driver circuit; acoloring layer over the second adhesive layer; a second insulating layerover the coloring layer; a third adhesive layer over the secondinsulating layer; a second flexible substrate over the third adhesivelayer, the second flexible substrate overlapping the first flexiblesubstrate with the display portion and the driver circuit positionedtherebetween; a conductive layer in and over an opening provided in thesecond adhesive layer, the coloring layer, the second insulating layer,the third adhesive layer, and the second flexible substrate; an externalelectrode electrically connected to the driver circuit via theconductive layer; and a first layer covering a bottom surface and sidesurfaces of the first flexible substrate, and a top surface and sidesurfaces of the second flexible substrate, wherein a Young's modulus ofthe first layer is smaller than a Young's modulus of each of the firstflexible substrate and the second flexible substrate.
 2. Thesemiconductor device according to claim 1, wherein a first part of theexternal electrode is covered with the first layer and a second part ofthe external electrode is not covered with the first layer.
 3. Thesemiconductor device according to claim 1, wherein the Young's modulusof the first layer is less than or equal to one fiftieth of the Young'smodulus of each of the first flexible substrate and the second flexiblesubstrate.
 4. The semiconductor device according to claim 1, wherein theYoung's modulus of each of the first flexible substrate and the secondflexible substrate is larger than or equal to 2 GPa and smaller than orequal to 20 GPa.
 5. The semiconductor device according to claim 1,wherein the first layer has a light-transmitting property.
 6. Thesemiconductor device according to claim 1, wherein the first layer isseamless.
 7. The semiconductor device according to claim 1, wherein thefirst layer comprises one of silicone rubber and fluorine rubber.
 8. Thesemiconductor device according to claim 1, wherein the display portioncomprises a transistor, and wherein a channel formation region of thetransistor comprises an oxide semiconductor.
 9. The semiconductor deviceaccording to claim 1, wherein the external electrode comprises aflexible printed circuit, and wherein the external electrode projectsfrom the first layer in a direction parallel to a long side of thesecond flexible substrate.
 10. An electronic device comprising thesemiconductor device according to claim 1, wherein the electronic devicefurther comprises at least one of an antenna, a battery, an operationswitch, a microphone, and a speaker.
 11. The semiconductor deviceaccording to claim 1, further comprising a touch sensor provided betweenthe display portion and the second flexible substrate.
 12. Thesemiconductor device according to claim 1, further comprising a touchsensor provided over the second flexible substrate.
 13. A semiconductordevice comprising: a first flexible substrate; a first adhesive layerover the first flexible substrate; a first insulating layer over thefirst adhesive layer; a display portion having a light-emitting elementover the first insulating layer; a driver circuit over the firstinsulating layer; a second adhesive layer over the display portion andthe driver circuit; a coloring layer over the second adhesive layer; asecond insulating layer over the coloring layer; a third adhesive layerover the second insulating layer; a second flexible substrate over thethird adhesive layer, the second flexible substrate overlapping thefirst flexible substrate with the display portion and the driver circuitpositioned therebetween; a conductive layer in and over an openingprovided in the second adhesive layer, the coloring layer, the secondinsulating layer, the third adhesive layer and the second flexiblesubstrate; an external electrode electrically connected to the drivercircuit via the conductive layer; and a first layer covering a firstregion of a bottom surface of the first flexible substrate, a topsurface of the second flexible substrate, and side surfaces of at leastone of the first flexible substrate and the second flexible substrate,wherein a Young's modulus of the first layer is smaller than a Young'smodulus of each of the first flexible substrate and the second flexiblesubstrate.
 14. The semiconductor device according to claim 13, wherein afirst part of the external electrode is covered with the first layer anda second part of the external electrode is not covered with the firstlayer.
 15. The semiconductor device according to claim 13, wherein theYoung's modulus of each of the first flexible substrate and the secondflexible substrate is larger than or equal to 2 GPa and smaller than orequal to 20 GPa.
 16. The semiconductor device according to claim 13,wherein the Young's modulus of the first layer is less than or equal toone fiftieth of the Young's modulus of each of the first flexiblesubstrate and the second flexible substrate.
 17. The semiconductordevice according to claim 13, wherein the display portion comprises atransistor, and wherein a channel formation region of the transistorcomprises an oxide semiconductor.
 18. The semiconductor device accordingto claim 13, wherein the first layer has a light-transmitting property.19. The semiconductor device according to claim 13, wherein the firstlayer is seamless.
 20. The semiconductor device according to claim 13,wherein the first layer comprises one of silicone rubber and fluorinerubber.
 21. The semiconductor device according to claim 13, wherein theexternal electrode comprises a flexible printed circuit, and wherein theexternal electrode projects from the first layer in a direction parallelto a long side of the second flexible substrate.
 22. An electronicdevice comprising the semiconductor device according to claim 13,wherein the electronic device further comprises at least one of anantenna, a battery, an operation switch, a microphone, and a speaker.23. The semiconductor device according to claim 13, further comprising atouch sensor provided between the display portion and the secondflexible substrate.
 24. The semiconductor device according to claim 13,further comprising a touch sensor provided over the second flexiblesubstrate.